This page lists diagnostic messages produced by the Dart analyzer, with details about what those messages mean and how you can fix your code. For more information about the analyzer, see Customizing static analysis.

The analyzer produces the following diagnostics for code that doesn't conform to the language specification or that might work in unexpected ways.

Classes extending 'AbiSpecificInteger' must have exactly one const constructor, no other members, and no type parameters.

The analyzer produces this diagnostic when a class that extends AbiSpecificInteger doesn't meet all of the following requirements:

• there must be exactly one constructor
• the constructor must be marked const
• there must not be any members of other than the one constructor
• there must not be any type parameters

The following code produces this diagnostic because the class C doesn't define a const constructor:

The following code produces this diagnostic because the constructor isn't a const constructor:

The following code produces this diagnostic because the class C defines multiple constructors:

The following code produces this diagnostic because the class C defines a field:

The following code produces this diagnostic because the class C has a type parameter:

Change the class so that it meets the requirements of having no type parameters and a single member that is a const constructor:

Classes extending 'AbiSpecificInteger' must have exactly one 'AbiSpecificIntegerMapping' annotation specifying the mapping from ABI to a 'NativeType' integer with a fixed size.

The analyzer produces this diagnostic when a class that extends AbiSpecificInteger has more than one AbiSpecificIntegerMapping annotation.

The following code produces this diagnostic because there are two AbiSpecificIntegerMapping annotations on the class C:

Remove all but one of the annotations, merging the arguments as appropriate:

Classes extending 'AbiSpecificInteger' must have exactly one 'AbiSpecificIntegerMapping' annotation specifying the mapping from ABI to a 'NativeType' integer with a fixed size.

The analyzer produces this diagnostic when a class that extends AbiSpecificInteger doesn't have an AbiSpecificIntegerMapping annotation.

The following code produces this diagnostic because there's no AbiSpecificIntegerMapping annotation on the class C:

Add an AbiSpecificIntegerMapping annotation to the class:

Invalid mapping to '{0}'; only mappings to 'Int8', 'Int16', 'Int32', 'Int64', 'Uint8', 'Uint16', 'UInt32', and 'Uint64' are supported.

The analyzer produces this diagnostic when a value in the map argument of an AbiSpecificIntegerMapping annotation is anything other than one of the following integer types:

• Int8
• Int16
• Int32
• Int64
• Uint8
• Uint16
• UInt32
• Uint64

The following code produces this diagnostic because the value of the map entry is Array<Uint8>, which isn't a valid integer type:

Use one of the valid types as a value in the map:

Abstract fields can't have initializers.

The analyzer produces this diagnostic when a field that has the abstract modifier also has an initializer.

The following code produces this diagnostic because f is marked as abstract and has an initializer:

The following code produces this diagnostic because f is marked as abstract and there's an initializer in the constructor:

If the field must be abstract, then remove the initializer:

If the field isn't required to be abstract, then remove the keyword:

A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.

The analyzer produces this diagnostic when a class is declared using both the modifier abstract and the modifier sealed. Sealed classes are implicitly abstract, so explicitly using both modifiers is not allowed.

The following code produces this diagnostic because the class C is declared using both abstract and sealed:

If the class should be abstract but not sealed, then remove the sealed modifier:

If the class should be both abstract and sealed, then remove the abstract modifier:

The {0} '{1}' is always abstract in the supertype.

The analyzer produces this diagnostic when an inherited member is referenced using super, but there is no concrete implementation of the member in the superclass chain. Abstract members can't be invoked.

The following code produces this diagnostic because B doesn't inherit a concrete implementation of a:

Remove the invocation of the abstract member, possibly replacing it with an invocation of a concrete member.

The name '{0}' is defined in the libraries '{1}' and '{2}'.

The analyzer produces this diagnostic when two or more export directives cause the same name to be exported from multiple libraries.

Given a file a.dart containing

And a file b.dart containing

The following code produces this diagnostic because the name C is being exported from both a.dart and b.dart:

If none of the names in one of the libraries needs to be exported, then remove the unnecessary export directives:

If all of the export directives are needed, then hide the name in all except one of the directives:

A member named '{0}' is defined in '{1}' and '{2}', and neither is more specific.

A member named '{0}' is defined in {1}, and none are more specific.

When code refers to a member of an object (for example, o.m() or o.m or o[i]) where the static type of o doesn't declare the member (m or [], for example), then the analyzer tries to find the member in an extension. For example, if the member is m, then the analyzer looks for extensions that declare a member named m and have an extended type that the static type of o can be assigned to. When there's more than one such extension in scope, the extension whose extended type is most specific is selected.

The analyzer produces this diagnostic when none of the extensions has an extended type that's more specific than the extended types of all of the other extensions, making the reference to the member ambiguous.

The following code produces this diagnostic because there's no way to choose between the member in E1 and the member in E2:

If you don't need both extensions, then you can delete or hide one of them.

If you need both, then explicitly select the one you want to use by using an extension override:

The name '{0}' is defined in the libraries {1}.

The analyzer produces this diagnostic when a name is referenced that is declared in two or more imported libraries.

Given a library (a.dart) that defines a class (C in this example):

And a library (b.dart) that defines a different class with the same name:

The following code produces this diagnostic:

If any of the libraries aren't needed, then remove the import directives for them:

If the name is still defined by more than one library, then add a hide clause to the import directives for all except one library:

If you must be able to reference more than one of these types, then add a prefix to each of the import directives, and qualify the references with the appropriate prefix:

The literal can't be either a map or a set because it contains at least one literal map entry or a spread operator spreading a 'Map', and at least one element which is neither of these.

Because map and set literals use the same delimiters ({ and }), the analyzer looks at the type arguments and the elements to determine which kind of literal you meant. When there are no type arguments, then the analyzer uses the types of the elements. If all of the elements are literal map entries and all of the spread operators are spreading a Map then it's a Map. If none of the elements are literal map entries and all of the spread operators are spreading an Iterable, then it's a Set. If neither of those is true then it's ambiguous.

The analyzer produces this diagnostic when at least one element is a literal map entry or a spread operator spreading a Map, and at least one element is neither of these, making it impossible for the analyzer to determine whether you are writing a map literal or a set literal.

The following code produces this diagnostic:

The list b can only be spread into a set, and the maps a and c can only be spread into a map, and the literal can't be both.

There are two common ways to fix this problem. The first is to remove all of the spread elements of one kind or another, so that the elements are consistent. In this case, that likely means removing the list and deciding what to do about the now unused parameter:

The second fix is to change the elements of one kind into elements that are consistent with the other elements. For example, you can add the elements of the list as keys that map to themselves:

This literal must be either a map or a set, but the elements don't have enough information for type inference to work.

Because map and set literals use the same delimiters ({ and }), the analyzer looks at the type arguments and the elements to determine which kind of literal you meant. When there are no type arguments and all of the elements are spread elements (which are allowed in both kinds of literals) then the analyzer uses the types of the expressions that are being spread. If all of the expressions have the type Iterable, then it's a set literal; if they all have the type Map, then it's a map literal.

This diagnostic is produced when none of the expressions being spread have a type that allows the analyzer to decide whether you were writing a map literal or a set literal.

The following code produces this diagnostic:

The problem occurs because there are no type arguments, and there is no information about the type of either a or b.

There are three common ways to fix this problem. The first is to add type arguments to the literal. For example, if the literal is intended to be a map literal, you might write something like this:

The second fix is to add type information so that the expressions have either the type Iterable or the type Map. You can add an explicit cast or, in this case, add types to the declarations of the two parameters:

The third fix is to add context information. In this case, that means adding a return type to the function:

In other cases, you might add a type somewhere else. For example, say the original code looks like this:

You might add a type annotation on x, like this:

Fields in a struct class whose type is 'Pointer' shouldn't have any annotations.

The analyzer produces this diagnostic when a field that's declared in a subclass of Struct and has the type Pointer also has an annotation associated with it.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field p, which has the type Pointer and is declared in a subclass of Struct, has the annotation @Double():

Remove the annotations from the field:

Argument '{0}' must be a constant.

The analyzer produces this diagnostic when an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction has an isLeaf argument whose value isn't a constant expression.

The analyzer also produces this diagnostic when an invocation of either Pointer.fromFunction or NativeCallable.isolateLocal has an exceptionalReturn argument whose value isn't a constant expression.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the value of the isLeaf argument is a parameter, and hence isn't a constant:

If there's a suitable constant that can be used, then replace the argument with a constant:

If there isn't a suitable constant, then replace the argument with a boolean literal:

Argument to 'Native.addressOf' must be annotated with @Native

The analyzer produces this diagnostic when the argument passed to Native.addressOf isn't annotated with the Native annotation.

The following code produces this diagnostic because the argument to addressOf is a string, not a field, and strings can't be annotated:

The following code produces this diagnostic because the function f is being passed to addressOf but isn't annotated as being Native:

If the argument isn't either a field or a function, then replace the argument with a field or function that's annotated with Native:

If the argument is either a field or a function, then annotate the field of function with Native:

The argument type '{0}' can't be assigned to the parameter type '{1}'. {2}

The analyzer produces this diagnostic when the static type of an argument can't be assigned to the static type of the corresponding parameter.

The following code produces this diagnostic because a num can't be assigned to a String:

If possible, rewrite the code so that the static type is assignable. In the example above you might be able to change the type of the parameter y:

If that fix isn't possible, then add code to handle the case where the argument value isn't the required type. One approach is to coerce other types to the required type:

Another approach is to add explicit type tests and fallback code:

If you believe that the runtime type of the argument will always be the same as the static type of the parameter, and you're willing to risk having an exception thrown at runtime if you're wrong, then add an explicit cast:

The argument type '{0}' can't be assigned to the parameter type '{1} Function(Object)' or '{1} Function(Object, StackTrace)'.

The analyzer produces this diagnostic when an invocation of Future.catchError has an argument that is a function whose parameters aren't compatible with the arguments that will be passed to the function when it's invoked. The static type of the first argument to catchError is just Function, even though the function that is passed in is expected to have either a single parameter of type Object or two parameters of type Object and StackTrace.

The following code produces this diagnostic because the closure being passed to catchError doesn't take any parameters, but the function is required to take at least one parameter:

The following code produces this diagnostic because the closure being passed to catchError takes three parameters, but it can't have more than two required parameters:

The following code produces this diagnostic because even though the closure being passed to catchError takes one parameter, the closure doesn't have a type that is compatible with Object:

Change the function being passed to catchError so that it has either one or two required parameters, and the parameters have the required types:

A redirecting constructor can't have an 'assert' initializer.

The analyzer produces this diagnostic when a redirecting constructor (a constructor that redirects to another constructor in the same class) has an assert in the initializer list.

The following code produces this diagnostic because the unnamed constructor is a redirecting constructor and also has an assert in the initializer list:

If the assert isn't needed, then remove it:

If the assert is needed, then convert the constructor into a factory constructor:

The asset directory '{0}' doesn't exist.

The analyzer produces this diagnostic when an asset list contains a value referencing a directory that doesn't exist.

Assuming that the directory assets doesn't exist, the following code produces this diagnostic because it's listed as a directory containing assets:

If the path is correct, then create a directory at that path.

If the path isn't correct, then change the path to match the path of the directory containing the assets.

The asset file '{0}' doesn't exist.

The analyzer produces this diagnostic when an asset list contains a value referencing a file that doesn't exist.

Assuming that the file doesNotExist.gif doesn't exist, the following code produces this diagnostic because it's listed as an asset:

If the path is correct, then create a file at that path.

If the path isn't correct, then change the path to match the path of the file containing the asset.

The value of the 'assets' field is expected to be a list of relative file paths.

The analyzer produces this diagnostic when the value of the assets key isn't a list.

The following code produces this diagnostic because the value of the assets key is a string when a list is expected:

Change the value of the asset list so that it's a list:

Asset map entry must contain a 'path' field.

The analyzer produces this diagnostic when an asset map is missing a path value.

The following code produces this diagnostic because the asset map is missing a path value:

Change the asset map so that it contains a path field with a string value (a valid POSIX-style file path):

Assets are required to be file paths (strings).

The analyzer produces this diagnostic when an assets list contains a value that isn't a string.

The following code produces this diagnostic because the assets list contains a map:

Change the assets list so that it only contains valid POSIX-style file paths:

An asset value is required to be a file path (string) or map.

The analyzer produces this diagnostic when an asset value isn't a string or a map.

The following code produces this diagnostic because the asset value is a list:

If you need to specify more than just the path to the asset, then replace the value with a map with a path key (a valid POSIX-style file path):

If you only need to specify the path, then replace the value with the path to the asset (a valid POSIX-style file path):

Asset paths are required to be file paths (strings).

The analyzer produces this diagnostic when an asset map contains a path value that isn't a string.

The following code produces this diagnostic because the asset map contains a path value which is a list:

Change the asset map so that it contains a path value which is a string (a valid POSIX-style file path):

'{0}' is marked 'doNotStore' and shouldn't be assigned to a field or top-level variable.

The analyzer produces this diagnostic when the value of a function (including methods and getters) that is explicitly or implicitly marked by the doNotStore annotation is stored in either a field or top-level variable.

The following code produces this diagnostic because the value of the function f is being stored in the top-level variable x:

Replace references to the field or variable with invocations of the function producing the value.

Constant variables can't be assigned a value after initialization.

The analyzer produces this diagnostic when it finds an assignment to a top-level variable, a static field, or a local variable that has the const modifier. The value of a compile-time constant can't be changed at runtime.

The following code produces this diagnostic because c is being assigned a value even though it has the const modifier:

If the variable must be assignable, then remove the const modifier:

If the constant shouldn't be changed, then either remove the assignment or use a local variable in place of references to the constant:

'{0}' can't be used as a setter because it's final.

The analyzer produces this diagnostic when it finds an invocation of a setter, but there's no setter because the field with the same name was declared to be final or const.

The following code produces this diagnostic because v is final:

If you need to be able to set the value of the field, then remove the modifier final from the field:

The final variable '{0}' can only be set once.

The analyzer produces this diagnostic when a local variable that was declared to be final is assigned after it was initialized.

The following code produces this diagnostic because x is final, so it can't have a value assigned to it after it was initialized:

Remove the keyword final, and replace it with var if there's no type annotation:

There isn't a setter named '{0}' in class '{1}'.

The analyzer produces this diagnostic when a reference to a setter is found; there is no setter defined for the type; but there is a getter defined with the same name.

The following code produces this diagnostic because there is no setter named x in C, but there is a getter named x:

If you want to invoke an existing setter, then correct the name:

If you want to invoke the setter but it just doesn't exist yet, then declare it:

Functions can't be assigned a value.

The analyzer produces this diagnostic when the name of a function appears on the left-hand side of an assignment expression.

The following code produces this diagnostic because the assignment to the function f is invalid:

If the right-hand side should be assigned to something else, such as a local variable, then change the left-hand side:

If the intent is to change the implementation of the function, then define a function-valued variable instead of a function:

Methods can't be assigned a value.

The analyzer produces this diagnostic when the target of an assignment is a method.

The following code produces this diagnostic because f can't be assigned a value because it's a method:

Rewrite the code so that there isn't an assignment to a method.

Types can't be assigned a value.

The analyzer produces this diagnostic when the name of a type name appears on the left-hand side of an assignment expression.

The following code produces this diagnostic because the assignment to the class C is invalid:

If the right-hand side should be assigned to something else, such as a local variable, then change the left-hand side:

The async for-in loop can only be used in an async function.

The analyzer produces this diagnostic when an async for-in loop is found in a function or method whose body isn't marked as being either async or async*.

The following code produces this diagnostic because the body of f isn't marked as being either async or async*, but f contains an async for-in loop:

If the function should return a Future, then mark the body with async:

If the function should return a Stream of values, then mark the body with async*:

If the function should be synchronous, then remove the await before the loop:

The 'await' expression can't be used in a 'late' local variable's initializer.

The analyzer produces this diagnostic when a local variable that has the late modifier uses an await expression in the initializer.

The following code produces this diagnostic because an await expression is used in the initializer for v, a local variable that is marked late:

If the initializer can be rewritten to not use await, then rewrite it:

If the initializer can't be rewritten, then remove the late modifier:

The 'await' expression can't be used for an expression with an extension type that is not a subtype of 'Future'.

The analyzer produces this diagnostic when the type of the expression in an await expression is an extension type, and the extension type isn't a subclass of Future.

The following code produces this diagnostic because the extension type E isn't a subclass of Future:

If the extension type is correctly defined, then remove the await:

If the extension type is intended to be awaitable, then add Future (or a subtype of Future) to the implements clause (adding an implements clause if there isn't one already), and make the representation type match:

The body might complete normally, causing 'null' to be returned, but the return type, '{0}', is a potentially non-nullable type.

The analyzer produces this diagnostic when a method or function has a return type that's potentially non-nullable but would implicitly return null if control reached the end of the function.

The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but the method is declared to not return null:

The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but because the class C can be instantiated with a non-nullable type argument, the method is effectively declared to not return null:

If there's a reasonable value that can be returned, then add a return statement at the end of the method:

If the method won't reach the implicit return, then add a throw at the end of the method:

If the method intentionally returns null at the end, then add an explicit return of null at the end of the method and change the return type so that it's valid to return null:

This 'onError' handler must return a value assignable to '{0}', but ends without returning a value.

The analyzer produces this diagnostic when the closure passed to the onError parameter of the Future.catchError method is required to return a non-null value (because of the Futures type argument) but can implicitly return null.

The following code produces this diagnostic because the closure passed to the catchError method is required to return an int but doesn't end with an explicit return, causing it to implicitly return null:

If the closure should sometimes return a non-null value, then add an explicit return to the closure:

If the closure should always return null, then change the type argument of the Future to be either void or Null:

This function has a nullable return type of '{0}', but ends without returning a value.

The analyzer produces this diagnostic when a method or function can implicitly return null by falling off the end. While this is valid Dart code, it's better for the return of null to be explicit.

The following code produces this diagnostic because the function f implicitly returns null:

If the return of null is intentional, then make it explicit:

If the function should return a non-null value along that path, then add the missing return statement:

A break label resolves to the 'case' or 'default' statement.

The analyzer produces this diagnostic when a break in a case clause inside a switch statement has a label that is associated with another case clause.

The following code produces this diagnostic because the label l is associated with the case clause for 0:

If the intent is to transfer control to the statement after the switch, then remove the label from the break statement:

If the intent is to transfer control to a different case block, then use continue rather than break:

The built-in identifier '{0}' can't be used as a type.

The analyzer produces this diagnostic when a built-in identifier is used where a type name is expected.

The following code produces this diagnostic because import can't be used as a type because it's a built-in identifier:

Replace the built-in identifier with the name of a valid type:

The built-in identifier '{0}' can't be used as a prefix name.

The built-in identifier '{0}' can't be used as a type name.

The built-in identifier '{0}' can't be used as a type parameter name.

The built-in identifier '{0}' can't be used as a typedef name.

The built-in identifier '{0}' can't be used as an extension name.

The built-in identifier '{0}' can't be used as an extension type name.

The analyzer produces this diagnostic when the name used in the declaration of a class, extension, mixin, typedef, type parameter, or import prefix is a built-in identifier. Built-in identifiers can't be used to name any of these kinds of declarations.

The following code produces this diagnostic because mixin is a built-in identifier:

Choose a different name for the declaration.

The last statement of the 'case' should be 'break', 'continue', 'rethrow', 'return', or 'throw'.

The analyzer produces this diagnostic when the last statement in a case block isn't one of the required terminators: break, continue, rethrow, return, or throw.

The following code produces this diagnostic because the case block ends with an assignment:

Add one of the required terminators:

The switch case expression type '{0}' can't override the '==' operator.

The analyzer produces this diagnostic when the type of the expression following the keyword case has an implementation of the == operator other than the one in Object.

The following code produces this diagnostic because the expression following the keyword case (C(0)) has the type C, and the class C overrides the == operator:

If there isn't a strong reason not to do so, then rewrite the code to use an if-else structure:

If you can't rewrite the switch statement and the implementation of == isn't necessary, then remove it:

If you can't rewrite the switch statement and you can't remove the definition of ==, then find some other value that can be used to control the switch:

The switch case expression type '{0}' must be a subtype of the switch expression type '{1}'.

The analyzer produces this diagnostic when the expression following case in a switch statement has a static type that isn't a subtype of the static type of the expression following switch.

The following code produces this diagnostic because 1 is an int, which isn't a subtype of String (the type of s):

If the value of the case expression is wrong, then change the case expression so that it has the required type:

If the value of the case expression is correct, then change the switch expression to have the required type:

This cast will always throw an exception because the nullable local variable '{0}' is not assigned.

The analyzer produces this diagnostic when a local variable that has a nullable type hasn't been assigned and is cast to a non-nullable type. Because the variable hasn't been assigned it has the default value of null, causing the cast to throw an exception.

The following code produces this diagnostic because the variable x is cast to a non-nullable type (int) when it's known to have the value null:

If the variable is expected to have a value before the cast, then add an initializer or an assignment:

If the variable isn't expected to be assigned, then remove the cast:

This cast always throws an exception because the expression always evaluates to 'null'.

The analyzer produces this diagnostic when an expression whose type is Null is being cast to a non-nullable type.

The following code produces this diagnostic because n is known to always be null, but it's being cast to a non-nullable type:

Remove the unnecessary cast:

The name '{0}' isn't a type, so it can't be used in an 'as' expression.

The analyzer produces this diagnostic when the name following the as in a cast expression is defined to be something other than a type.

The following code produces this diagnostic because x is a variable, not a type:

Replace the name with the name of a type:

The class '{0}' can't be used as a mixin because it's neither a mixin class nor a mixin.

The analyzer produces this diagnostic when a class that is neither a mixin class nor a mixin is used in a with clause.

The following code produces this diagnostic because the class M is being used as a mixin, but it isn't defined as a mixin class:

If the class can be a pure mixin, then change class to mixin:

If the class needs to be both a class and a mixin, then add mixin:

Constant values from a deferred library can't be used as keys in a 'const' map literal.

Constant values from a deferred library can't be used as values in a 'const' constructor.

Constant values from a deferred library can't be used as values in a 'const' list literal.

Constant values from a deferred library can't be used as values in a 'const' map literal.

Constant values from a deferred library can't be used as values in a 'const' set literal.

The analyzer produces this diagnostic when a collection literal that is either explicitly (because it's prefixed by the const keyword) or implicitly (because it appears in a constant context) a constant contains a value that is declared in a library that is imported using a deferred import. Constants are evaluated at compile time, and values from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Given a file a.dart that defines the constant zero:

The following code produces this diagnostic because the constant list literal contains a.zero, which is imported using a deferred import:

If the collection literal isn't required to be constant, then remove the const keyword:

If the collection is required to be constant and the imported constant must be referenced, then remove the keyword deferred from the import:

If you don't need to reference the constant, then replace it with a suitable value:

The class '{0}' can't implement Finalizable.

The analyzer produces this diagnostic when a subclass of either Struct or Union implements Finalizable.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class S implements Finalizable:

Try removing the implements clause from the class:

Concrete classes can't have 'Enum' as a superinterface.

The analyzer produces this diagnostic when a concrete class indirectly has the class Enum as a superinterface.

The following code produces this diagnostic because the concrete class B has Enum as a superinterface as a result of implementing A:

If the implemented class isn't the class you intend to implement, then change it:

If the implemented class can be changed to not implement Enum, then do so:

If the implemented class can't be changed to not implement Enum, then remove it from the implements clause:

'{0}' must have a method body because '{1}' isn't abstract.

The analyzer produces this diagnostic when a member of a concrete class is found that doesn't have a concrete implementation. Concrete classes aren't allowed to contain abstract members.

The following code produces this diagnostic because m is an abstract method but C isn't an abstract class:

If it's valid to create instances of the class, provide an implementation for the member:

If it isn't valid to create instances of the class, mark the class as being abstract:

'{0}' can't be used to name both a constructor and a static field in this class.

'{0}' can't be used to name both a constructor and a static getter in this class.

'{0}' can't be used to name both a constructor and a static method in this class.

'{0}' can't be used to name both a constructor and a static setter in this class.

The analyzer produces this diagnostic when a named constructor and either a static method or static field have the same name. Both are accessed using the name of the class, so having the same name makes the reference ambiguous.

The following code produces this diagnostic because the static field foo and the named constructor foo have the same name:

The following code produces this diagnostic because the static method foo and the named constructor foo have the same name:

Rename either the member or the constructor.

The {0} '{1}' can't implement both '{2}' and '{3}' because the type arguments are different.

The analyzer produces this diagnostic when a class attempts to implement a generic interface multiple times, and the values of the type arguments aren't the same.

The following code produces this diagnostic because C is defined to implement both I<int> (because it extends A) and I<String> (because it implementsB), but int and String aren't the same type:

Rework the type hierarchy to avoid this situation. For example, you might make one or both of the inherited types generic so that C can specify the same type for both type arguments:

'{0}' can't be used to name both a type parameter and the class in which the type parameter is defined.

'{0}' can't be used to name both a type parameter and the enum in which the type parameter is defined.

'{0}' can't be used to name both a type parameter and the extension in which the type parameter is defined.

'{0}' can't be used to name both a type parameter and the extension type in which the type parameter is defined.

'{0}' can't be used to name both a type parameter and the mixin in which the type parameter is defined.

The analyzer produces this diagnostic when a class, mixin, or extension declaration declares a type parameter with the same name as the class, mixin, or extension that declares it.

The following code produces this diagnostic because the type parameter C has the same name as the class C of which it's a part:

Rename either the type parameter, or the class, mixin, or extension:

'{0}' can't be used to name both a type parameter and a member in this class.

'{0}' can't be used to name both a type parameter and a member in this enum.

'{0}' can't be used to name both a type parameter and a member in this extension type.

'{0}' can't be used to name both a type parameter and a member in this extension.

'{0}' can't be used to name both a type parameter and a member in this mixin.

The analyzer produces this diagnostic when a class, mixin, or extension declaration declares a type parameter with the same name as one of the members of the class, mixin, or extension that declares it.

The following code produces this diagnostic because the type parameter T has the same name as the field T:

Rename either the type parameter or the member with which it conflicts:

The matched value type '{0}' can never be equal to this constant of type '{1}'.

The analyzer produces this diagnostic when a constant pattern can never match the value it's being tested against because the type of the constant is known to never match the type of the value.

The following code produces this diagnostic because the type of the constant pattern (true) is bool, and the type of the value being matched (x) is int, and a Boolean can never match an integer:

If the type of the value is correct, then rewrite the pattern to be compatible:

If the type of the constant is correct, then rewrite the value to be compatible:

The expression of a constant pattern must be a valid constant.

The analyzer produces this diagnostic when a constant pattern has an expression that isn't a valid constant.

The following code produces this diagnostic because the constant pattern i isn't a constant:

If the value that should be matched is known, then replace the expression with a constant:

If the value that should be matched isn't known, then rewrite the code to not use a pattern:

A value of type '{0}' can't be assigned to a parameter of type '{1}' in a const constructor.

The analyzer produces this diagnostic when the runtime type of a constant value can't be assigned to the static type of a constant constructor's parameter.

The following code produces this diagnostic because the runtime type of i is int, which can't be assigned to the static type of s:

Pass a value of the correct type to the constructor:

Can't define the 'const' constructor because the field '{0}' is initialized with a non-constant value.

The analyzer produces this diagnostic when a constructor has the keyword const, but a field in the class is initialized to a non-constant value.

The following code produces this diagnostic because the field s is initialized to a non-constant value:

If the field can be initialized to a constant value, then change the initializer to a constant expression:

If the field can't be initialized to a constant value, then remove the keyword const from the constructor:

A constant constructor can't call a non-constant super constructor of '{0}'.

The analyzer produces this diagnostic when a constructor that is marked as const invokes a constructor from its superclass that isn't marked as const.

The following code produces this diagnostic because the const constructor in B invokes the constructor nonConst from the class A, and the superclass constructor isn't a const constructor:

If it isn't essential to invoke the superclass constructor that is currently being invoked, then invoke a constant constructor from the superclass:

If it's essential that the current constructor be invoked and if you can modify it, then add const to the constructor in the superclass:

If it's essential that the current constructor be invoked and you can't modify it, then remove const from the constructor in the subclass:

Can't define a const constructor for a class with non-final fields.

The analyzer produces this diagnostic when a constructor is marked as a const constructor, but the constructor is defined in a class that has at least one non-final instance field (either directly or by inheritance).

The following code produces this diagnostic because the field x isn't final:

If it's possible to mark all of the fields as final, then do so:

If it isn't possible to mark all of the fields as final, then remove the keyword const from the constructor:

Deferred classes can't be created with 'const'.

The analyzer produces this diagnostic when a class from a library that is imported using a deferred import is used to create a const object. Constants are evaluated at compile time, and classes from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because it attempts to create a const instance of a class from a deferred library:

If the object isn't required to be a constant, then change the code so that a non-constant instance is created:

If the object must be a constant, then remove deferred from the import directive:

Const variables must be initialized with a constant value.

The analyzer produces this diagnostic when a value that isn't statically known to be a constant is assigned to a variable that's declared to be a const variable.

The following code produces this diagnostic because x isn't declared to be const:

If the value being assigned can be declared to be const, then change the declaration:

If the value can't be declared to be const, then remove the const modifier from the variable, possibly using final in its place:

Constant values from a deferred library can't be used to initialize a 'const' variable.

The analyzer produces this diagnostic when a const variable is initialized using a const variable from a library that is imported using a deferred import. Constants are evaluated at compile time, and values from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because the variable pi is being initialized using the constant math.pi from the library dart:math, and dart:math is imported as a deferred library:

If you need to reference the value of the constant from the imported library, then remove the keyword deferred:

If you don't need to reference the imported constant, then remove the reference:

Only static fields can be declared as const.

The analyzer produces this diagnostic when an instance field is marked as being const.

The following code produces this diagnostic because f is an instance field:

If the field needs to be an instance field, then remove the keyword const, or replace it with final:

If the field really should be a const field, then make it a static field:

The type of a key in a constant map can't override the '==' operator, or 'hashCode', but the class '{0}' does.

The analyzer produces this diagnostic when the class of object used as a key in a constant map literal implements either the == operator, the getter hashCode, or both. The implementation of constant maps uses both the == operator and the hashCode getter, so any implementation other than the ones inherited from Object requires executing arbitrary code at compile time, which isn't supported.

The following code produces this diagnostic because the constant map contains a key whose type is C, and the class C overrides the implementation of ==:

The following code produces this diagnostic because the constant map contains a key whose type is C, and the class C overrides the implementation of hashCode:

If you can remove the implementation of == and hashCode from the class, then do so:

If you can't remove the implementation of == and hashCode from the class, then make the map non-constant:

The constant '{0}' must be initialized.

The analyzer produces this diagnostic when a variable that is declared to be a constant doesn't have an initializer.

The following code produces this diagnostic because c isn't initialized:

Add an initializer:

(Previously known as const_set_element_type_implements_equals)

An element in a constant set can't override the '==' operator, or 'hashCode', but the type '{0}' does.

The analyzer produces this diagnostic when the class of object used as an element in a constant set literal implements either the == operator, the getter hashCode, or both. The implementation of constant sets uses both the == operator and the hashCode getter, so any implementation other than the ones inherited from Object requires executing arbitrary code at compile time, which isn't supported.

The following code produces this diagnostic because the constant set contains an element whose type is C, and the class C overrides the implementation of ==:

The following code produces this diagnostic because the constant set contains an element whose type is C, and the class C overrides the implementation of hashCode:

If you can remove the implementation of == and hashCode from the class, then do so:

If you can't remove the implementation of == and hashCode from the class, then make the set non-constant:

A list or a set is expected in this spread.

The analyzer produces this diagnostic when the expression of a spread operator in a constant list or set evaluates to something other than a list or a set.

The following code produces this diagnostic because the value of list1 is null, which is neither a list nor a set:

Change the expression to something that evaluates to either a constant list or a constant set:

A map is expected in this spread.

The analyzer produces this diagnostic when the expression of a spread operator in a constant map evaluates to something other than a map.

The following code produces this diagnostic because the value of map1 is null, which isn't a map:

Change the expression to something that evaluates to a constant map:

The constructor being called isn't a const constructor.

The analyzer produces this diagnostic when the keyword const is used to invoke a constructor that isn't marked with const.

The following code produces this diagnostic because the constructor in A isn't a const constructor:

If it's desirable and possible to make the class a constant class (by making all of the fields of the class, including inherited fields, final), then add the keyword const to the constructor:

Otherwise, remove the keyword const:

Arguments of a constant creation must be constant expressions.

The analyzer produces this diagnostic when a const constructor is invoked with an argument that isn't a constant expression.

The following code produces this diagnostic because i isn't a constant:

Either make all of the arguments constant expressions, or remove the const keyword to use the non-constant form of the constructor:

A constant constructor tearoff can't use a type parameter as a type argument.

A constant creation can't use a type parameter as a type argument.

A constant function tearoff can't use a type parameter as a type argument.

The analyzer produces this diagnostic when a type parameter is used as a type argument in a const invocation of a constructor. This isn't allowed because the value of the type parameter (the actual type that will be used at runtime) can't be known at compile time.

The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant:

If the type that will be used for the type parameter can be known at compile time, then remove the use of the type parameter:

If the type that will be used for the type parameter can't be known until runtime, then remove the keyword const:

(Previously known as continue_label_on_switch)

The label used in a 'continue' statement must be defined on either a loop or a switch member.

The analyzer produces this diagnostic when the label in a continue statement resolves to a label on a switch statement.

The following code produces this diagnostic because the label l, used to label a switch statement, is used in the continue statement:

Find a different way to achieve the control flow you need; for example, by introducing a loop that re-executes the switch statement.

Subclasses of 'Struct' and 'Union' are backed by native memory, and can't be instantiated by a generative constructor.

The analyzer produces this diagnostic when a subclass of either Struct or Union is instantiated using a generative constructor.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C is being instantiated using a generative constructor:

If you need to allocate the structure described by the class, then use the ffi package to do so:

The name '{0}' isn't a class.

The analyzer produces this diagnostic when an instance creation using either new or const specifies a name that isn't defined as a class.

The following code produces this diagnostic because f is a function rather than a class:

If a class should be created, then replace the invalid name with the name of a valid class:

If the name is the name of a function and you want that function to be invoked, then remove the new or const keyword:

Dead code.

Dead code: The assigned-to wildcard variable is marked late and can never be referenced so this initializer will never be evaluated.

The analyzer produces this diagnostic when code is found that won't be executed because execution will never reach the code.

The following code produces this diagnostic because the invocation of print occurs after the function has returned:

If the code isn't needed, then remove it:

If the code needs to be executed, then either move the code to a place where it will be executed:

Or, rewrite the code before it, so that it can be reached:

Dead code: Catch clauses after a 'catch (e)' or an 'on Object catch (e)' are never reached.

The analyzer produces this diagnostic when a catch clause is found that can't be executed because it's after a catch clause of the form catch (e) or on Object catch (e). The first catch clause that matches the thrown object is selected, and both of those forms will match any object, so no catch clauses that follow them will be selected.

The following code produces this diagnostic:

If the clause should be selectable, then move the clause before the general clause:

If the clause doesn't need to be selectable, then remove it:

Dead code: This on-catch block won't be executed because '{0}' is a subtype of '{1}' and hence will have been caught already.

The analyzer produces this diagnostic when a catch clause is found that can't be executed because it is after a catch clause that catches either the same type or a supertype of the clause's type. The first catch clause that matches the thrown object is selected, and the earlier clause always matches anything matchable by the highlighted clause, so the highlighted clause will never be selected.

The following code produces this diagnostic:

If the clause should be selectable, then move the clause before the general clause:

If the clause doesn't need to be selectable, then remove it:

The left operand can't be null, so the right operand is never executed.

The analyzer produces this diagnostic in two cases.

The first is when the left operand of an ?? operator can't be null. The right operand is only evaluated if the left operand has the value null, and because the left operand can't be null, the right operand is never evaluated.

The second is when the left-hand side of an assignment using the ??= operator can't be null. The right-hand side is only evaluated if the left-hand side has the value null, and because the left-hand side can't be null, the right-hand side is never evaluated.

The following code produces this diagnostic because x can't be null:

The following code produces this diagnostic because f can't be null:

If the diagnostic is reported for an ?? operator, then remove the ?? operator and the right operand:

If the diagnostic is reported for an assignment, and the assignment isn't needed, then remove the assignment:

If the assignment is needed, but should be based on a different condition, then rewrite the code to use = and the different condition:

The default 'List' constructor isn't available when null safety is enabled.

The analyzer produces this diagnostic when it finds a use of the default constructor for the class List in code that has opted in to null safety.

Assuming the following code is opted in to null safety, it produces this diagnostic because it uses the default List constructor:

If no initial size is provided, then convert the code to use a list literal:

If an initial size needs to be provided and there is a single reasonable initial value for the elements, then use List.filled:

If an initial size needs to be provided but each element needs to be computed, then use List.generate:

Parameters in a function type can't have default values.

The analyzer produces this diagnostic when a function type associated with a parameter includes optional parameters that have a default value. This isn't allowed because the default values of parameters aren't part of the function's type, and therefore including them doesn't provide any value.

The following code produces this diagnostic because the parameter p has a default value even though it's part of the type of the parameter g:

Remove the default value from the function-type's parameter:

Default values aren't allowed in factory constructors that redirect to another constructor.

The analyzer produces this diagnostic when a factory constructor that redirects to another constructor specifies a default value for an optional parameter.

The following code produces this diagnostic because the factory constructor in A has a default value for the optional parameter x:

Remove the default value from the factory constructor:

Note that this fix might change the value used when the optional parameter is omitted. If that happens, and if that change is a problem, then consider making the optional parameter a required parameter in the factory method:

Required named parameters can't have a default value.

The analyzer produces this diagnostic when a named parameter has both the required modifier and a default value. If the parameter is required, then a value for the parameter is always provided at the call sites, so the default value can never be used.

The following code generates this diagnostic:

If the parameter is really required, then remove the default value:

If the parameter isn't always required, then remove the required modifier:

Imports of deferred libraries must hide all extensions.

The analyzer produces this diagnostic when a library that is imported using a deferred import declares an extension that is visible in the importing library. Extension methods are resolved at compile time, and extensions from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Given a file a.dart that defines a named extension:

The following code produces this diagnostic because the named extension is visible to the library:

If the library must be imported as deferred, then either add a show clause listing the names being referenced or add a hide clause listing all of the named extensions. Adding a show clause would look like this:

Adding a hide clause would look like this:

With the first fix, the benefit is that if new extensions are added to the imported library, then the extensions won't cause a diagnostic to be generated.

If the library doesn't need to be imported as deferred, or if you need to make use of the extension method declared in it, then remove the keyword deferred:

The late local variable '{0}' is definitely unassigned at this point.

The analyzer produces this diagnostic when definite assignment analysis shows that a local variable that's marked as late is read before being assigned.

The following code produces this diagnostic because x wasn't assigned a value before being read:

Assign a value to the variable before reading from it:

The value of the '{0}' field is expected to be a map.

The analyzer produces this diagnostic when the value of either the dependencies or dev_dependencies key isn't a map.

The following code produces this diagnostic because the value of the top-level dependencies key is a list:

Use a map as the value of the dependencies key:

Using a colon as the separator before a default value is deprecated and will not be supported in language version 3.0 and later.

The analyzer produces this diagnostic when a colon (:) is used as the separator before the default value of an optional named parameter. While this syntax is allowed, it is deprecated in favor of using an equal sign (=).

The following code produces this diagnostic because a colon is being used before the default value of the optional parameter i:

Replace the colon with an equal sign.

The ability to import '{0}' indirectly is deprecated.

The analyzer produces this diagnostic when one library imports a name from a second library, and the second library exports the name from a third library but has indicated that it won't export the third library in the future.

Given a library a.dart defining the class A:

And a second library b.dart that exports a.dart but has marked the export as being deprecated:

The following code produces this diagnostic because the class A won't be exported from b.dart in some future version:

If the name is available from a different library that you can import, then replace the existing import with an import for that library (or add an import for the defining library if you still need the old import):

If the name isn't available, then look for instructions from the library author or contact them directly to find out how to update your code.

The '{0}' field is no longer used and can be removed.

The analyzer produces this diagnostic when a key is used in a pubspec.yaml file that was deprecated. Unused keys take up space and might imply semantics that are no longer valid.

The following code produces this diagnostic because the author key is no longer being used:

Remove the deprecated key:

'{0}' is deprecated and shouldn't be used.

'{0}' is deprecated and shouldn't be used. {1}

The analyzer produces this diagnostic when a deprecated library or class member is used in a different package.

If the method m in the class C is annotated with @deprecated, then the following code produces this diagnostic:

The documentation for declarations that are annotated with @deprecated should indicate what code to use in place of the deprecated code.

'{0}' is deprecated and shouldn't be used.

'{0}' is deprecated and shouldn't be used. {1}

The analyzer produces this diagnostic when a deprecated library member or class member is used in the same package in which it's declared.

The following code produces this diagnostic because x is deprecated:

The fix depends on what's been deprecated and what the replacement is. The documentation for deprecated declarations should indicate what code to use in place of the deprecated code.

Using the 'new' keyword in a comment reference is deprecated.

The analyzer produces this diagnostic when a comment reference (the name of a declaration enclosed in square brackets in a documentation comment) uses the keyword new to refer to a constructor. This form is deprecated.

The following code produces this diagnostic because the unnamed constructor is being referenced using new C:

The following code produces this diagnostic because the constructor named c is being referenced using new C.c:

If you're referencing a named constructor, then remove the keyword new:

If you're referencing the unnamed constructor, then remove the keyword new and append .new after the class name:

Extending 'Function' is deprecated.

Implementing 'Function' has no effect.

Mixing in 'Function' is deprecated.

The analyzer produces this diagnostic when the class Function is used in either the extends, implements, or with clause of a class or mixin. Using the class Function in this way has no semantic value, so it's effectively dead code.

The following code produces this diagnostic because Function is used as the superclass of F:

Remove the class Function from whichever clause it's in, and remove the whole clause if Function is the only type in the clause:

Only a generic type, generic function, generic instance method, or generic constructor can have type arguments.

The analyzer produces this diagnostic when an expression with a value that is anything other than one of the allowed kinds of values is followed by type arguments. The allowed kinds of values are:

• generic types,
• generic constructors, and
• generic functions, including top-level functions, static and instance members, and local functions.

The following code produces this diagnostic because i is a top-level variable, which isn't one of the allowed cases:

If the referenced value is correct, then remove the type arguments:

The operator x ~/ y is more efficient than (x / y).toInt().

The analyzer produces this diagnostic when the result of dividing two numbers is converted to an integer using toInt. Dart has a built-in integer division operator that is both more efficient and more concise.

The following code produces this diagnostic because the result of dividing x and y is converted to an integer using toInt:

Use the integer division operator (~/):

The constructor with name '{0}' is already defined.

The unnamed constructor is already defined.

The analyzer produces this diagnostic when a class declares more than one unnamed constructor or when it declares more than one constructor with the same name.

The following code produces this diagnostic because there are two declarations for the unnamed constructor:

The following code produces this diagnostic because there are two declarations for the constructor named m:

If there are multiple unnamed constructors and all of the constructors are needed, then give all of them, or all except one of them, a name:

If there are multiple unnamed constructors and all except one of them are unneeded, then remove the constructors that aren't needed:

If there are multiple named constructors and all of the constructors are needed, then rename all except one of them:

If there are multiple named constructors and all except one of them are unneeded, then remove the constructors that aren't needed:

The name '{0}' is already defined.

The analyzer produces this diagnostic when a name is declared, and there is a previous declaration with the same name in the same scope.

The following code produces this diagnostic because the name x is declared twice:

Choose a different name for one of the declarations.

Duplicate export.

The analyzer produces this diagnostic when an export directive is found that is the same as an export before it in the file. The second export doesn't add value and should be removed.

The following code produces this diagnostic because the same library is being exported twice:

Remove the unnecessary export:

The field '{0}' can't be initialized by multiple parameters in the same constructor.

The analyzer produces this diagnostic when there's more than one initializing formal parameter for the same field in a constructor's parameter list. It isn't useful to assign a value that will immediately be overwritten.

The following code produces this diagnostic because this.f appears twice in the parameter list:

Remove one of the initializing formal parameters:

The field name '{0}' is already used in this record.

The analyzer produces this diagnostic when either a record literal or a record type annotation contains a field whose name is the same as a previously declared field in the same literal or type.

The following code produces this diagnostic because the record literal has two fields named a:

The following code produces this diagnostic because the record type annotation has two fields named a, one a positional field and the other a named field:

Rename one or both of the fields:

Duplicate hidden name.

The analyzer produces this diagnostic when a name occurs multiple times in a hide clause. Repeating the name is unnecessary.

The following code produces this diagnostic because the name min is hidden more than once:

If the name was mistyped in one or more places, then correct the mistyped names:

If the name wasn't mistyped, then remove the unnecessary name from the list:

The diagnostic '{0}' doesn't need to be ignored here because it's already being ignored.

The analyzer produces this diagnostic when a diagnostic name appears in an ignore comment, but the diagnostic is already being ignored, either because it's already included in the same ignore comment or because it appears in an ignore-in-file comment.

The following code produces this diagnostic because the diagnostic named unused_local_variable is already being ignored for the whole file so it doesn't need to be ignored on a specific line:

The following code produces this diagnostic because the diagnostic named unused_local_variable is being ignored twice on the same line:

Remove the ignore comment, or remove the unnecessary diagnostic name if the ignore comment is ignoring more than one diagnostic:

Duplicate import.

The analyzer produces this diagnostic when an import directive is found that is the same as an import before it in the file. The second import doesn't add value and should be removed.

The following code produces this diagnostic:

Remove the unnecessary import:

The argument for the named parameter '{0}' was already specified.

The analyzer produces this diagnostic when an invocation has two or more named arguments that have the same name.

The following code produces this diagnostic because there are two arguments with the name a:

If one of the arguments should have a different name, then change the name:

If one of the arguments is wrong, then remove it:

The library already contains a part with the URI '{0}'.

The analyzer produces this diagnostic when a single file is referenced in multiple part directives.

Given a file part.dart containing

The following code produces this diagnostic because the file part.dart is included multiple times:

Remove all except the first of the duplicated part directives:

The variable '{0}' is already assigned in this pattern.

The analyzer produces this diagnostic when a single pattern variable is assigned a value more than once in the same pattern assignment.

The following code produces this diagnostic because the variable a is assigned twice in the pattern (a, a):

If you need to capture all of the values, then use a unique variable for each of the subpatterns being matched:

If some of the values don't need to be captured, then use a wildcard pattern _ to avoid having to bind the value to a variable:

The field '{0}' is already matched in this pattern.

The analyzer produces this diagnostic when a record pattern matches the same field more than once, or when an object pattern matches the same getter more than once.

The following code produces this diagnostic because the record field a is matched twice in the same record pattern:

The following code produces this diagnostic because the getter f is matched twice in the same object pattern:

If the pattern should match for more than one value of the duplicated field, then use a logical-or pattern:

If the pattern should match against multiple fields, then change the name of one of the fields:

At most one rest element is allowed in a list or map pattern.

The analyzer produces this diagnostic when there's more than one rest pattern in either a list or map pattern. A rest pattern will capture any values unmatched by other subpatterns, making subsequent rest patterns unnecessary because there's nothing left to capture.

The following code produces this diagnostic because there are two rest patterns in the list pattern:

Remove all but one of the rest patterns:

Duplicate shown name.

The analyzer produces this diagnostic when a name occurs multiple times in a show clause. Repeating the name is unnecessary.

The following code produces this diagnostic because the name min is shown more than once:

If the name was mistyped in one or more places, then correct the mistyped names:

If the name wasn't mistyped, then remove the unnecessary name from the list:

The variable '{0}' is already defined in this pattern.

The analyzer produces this diagnostic when a branch of a logical-and pattern declares a variable that is already declared in an earlier branch of the same pattern.

The following code produces this diagnostic because the variable a is declared in both branches of the logical-and pattern:

If you need to capture the matched value in multiple branches, then change the names of the variables so that they are unique:

If you only need to capture the matched value on one branch, then remove the variable pattern from all but one branch:

A map pattern must have at least one entry.

The analyzer produces this diagnostic when a map pattern is empty.

The following code produces this diagnostic because the map pattern is empty:

If the pattern should match any map, then replace it with an object pattern:

If the pattern should only match an empty map, then check the length in the pattern:

A record literal without fields can't have a trailing comma.

The analyzer produces this diagnostic when a record literal that has no fields has a trailing comma. Empty record literals can't contain a comma.

The following code produces this diagnostic because the empty record literal has a trailing comma:

If the record is intended to be empty, then remove the comma:

If the record is intended to have one or more fields, then add the expressions used to compute the values of those fields:

The list of named fields in a record type can't be empty.

The analyzer produces this diagnostic when a record type has an empty list of named fields.

The following code produces this diagnostic because the record type has an empty list of named fields:

If the record is intended to have named fields, then add the types and names of the fields:

If the record isn't intended to have named fields, then remove the curly braces:

A record type without fields can't have a trailing comma.

The analyzer produces this diagnostic when a record type that has no fields has a trailing comma. Empty record types can't contain a comma.

The following code produces this diagnostic because the empty record type has a trailing comma:

If the record type is intended to be empty, then remove the comma:

If the record type is intended to have one or more fields, then add the types of those fields:

The class '{0}' can't be empty because it's a subclass of '{1}'.

The analyzer produces this diagnostic when a subclass of Struct or Union doesn't have any fields. Having an empty Struct or Union isn't supported.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C, which extends Struct, doesn't declare any fields:

If the class is intended to be a struct, then declare one or more fields:

If the class is intended to be used as a type argument to Pointer, then make it a subclass of Opaque:

If the class isn't intended to be a struct, then remove or change the extends clause:

The name of the enum value can't be the same as the enum's name.

The analyzer produces this diagnostic when an enum value has the same name as the enum in which it's declared.

The following code produces this diagnostic because the enum value E has the same name as the enclosing enum E:

If the name of the enum is correct, then rename the constant:

If the name of the constant is correct, then rename the enum:

The invoked constructor isn't a 'const' constructor.

The analyzer produces this diagnostic when an enum value is being created using either a factory constructor or a generative constructor that isn't marked as being const.

The following code produces this diagnostic because the enum value e is being initialized by a factory constructor:

Use a generative constructor marked as const:

Mixins applied to enums can't have instance variables.

The analyzer produces this diagnostic when a mixin that's applied to an enum declares one or more instance variables. This isn't allowed because the enum values are constant, and there isn't any way for the constructor in the enum to initialize any of the mixin's fields.

The following code produces this diagnostic because the mixin M defines the instance field x:

If you need to apply the mixin, then change all instance fields into getter and setter pairs and implement them in the enum if necessary:

If you don't need to apply the mixin, then remove it:

'{0}' must have a method body because '{1}' is an enum.

The analyzer produces this diagnostic when a member of an enum is found that doesn't have a concrete implementation. Enums aren't allowed to contain abstract members.

The following code produces this diagnostic because m is an abstract method and E is an enum:

Provide an implementation for the member:

The name 'values' is not a valid name for an enum.

The analyzer produces this diagnostic when an enum is declared to have the name values. This isn't allowed because the enum has an implicit static field named values, and the two would collide.

The following code produces this diagnostic because there's an enum declaration that has the name values:

Rename the enum to something other than values.

Two elements in a constant set literal can't be equal.

The analyzer produces this diagnostic when two elements in a constant set literal have the same value. The set can only contain each value once, which means that one of the values is unnecessary.

The following code produces this diagnostic because the string 'a' is specified twice:

Remove one of the duplicate values:

Note that literal sets preserve the order of their elements, so the choice of which element to remove might affect the order in which elements are returned by an iterator.

Two elements in a set literal shouldn't be equal.

The analyzer produces this diagnostic when an element in a non-constant set is the same as a previous element in the same set. If two elements are the same, then the second value is ignored, which makes having both elements pointless and likely signals a bug.

The following code produces this diagnostic because the element 1 appears twice:

If both elements should be included in the set, then change one of the elements:

If only one of the elements is needed, then remove the one that isn't needed:

Note that literal sets preserve the order of their elements, so the choice of which element to remove might affect the order in which elements are returned by an iterator.

Two keys in a constant map literal can't be equal.

The analyzer produces this diagnostic when a key in a constant map is the same as a previous key in the same map. If two keys are the same, then the second value would overwrite the first value, which makes having both pairs pointless.

The following code produces this diagnostic because the key 1 is used twice:

If both entries should be included in the map, then change one of the keys to be different:

If only one of the entries is needed, then remove the one that isn't needed:

Note that literal maps preserve the order of their entries, so the choice of which entry to remove might affect the order in which keys and values are returned by an iterator.

Two keys in a map literal shouldn't be equal.

The analyzer produces this diagnostic when a key in a non-constant map is the same as a previous key in the same map. If two keys are the same, then the second value overwrites the first value, which makes having both pairs pointless and likely signals a bug.

The following code produces this diagnostic because the keys a and b have the same value:

If both entries should be included in the map, then change one of the keys:

If only one of the entries is needed, then remove the one that isn't needed:

Note that literal maps preserve the order of their entries, so the choice of which entry to remove might affect the order in which the keys and values are returned by an iterator.

Two keys in a map pattern can't be equal.

The analyzer produces this diagnostic when a map pattern contains more than one key with the same name. The same key can't be matched twice.

The following code produces this diagnostic because the key 'a' appears twice:

If you are trying to match two different keys, then change one of the keys in the pattern:

If you are trying to match the same key, but allow any one of multiple patterns to match, the use a logical-or pattern:

List patterns require one type argument or none, but {0} found.

The analyzer produces this diagnostic when a list pattern has more than one type argument. List patterns can have either zero type arguments or one type argument, but can't have more than one.

The following code produces this diagnostic because the list pattern ([0]) has two type arguments:

Remove all but one of the type arguments:

List literals require one type argument or none, but {0} found.

The analyzer produces this diagnostic when a list literal has more than one type argument.

The following code produces this diagnostic because the list literal has two type arguments when it can have at most one:

Remove all except one of the type arguments:

Set literals require one type argument or none, but {0} were found.

The analyzer produces this diagnostic when a set literal has more than one type argument.

The following code produces this diagnostic because the set literal has three type arguments when it can have at most one:

Remove all except one of the type arguments:

Map patterns require two type arguments or none, but {0} found.

The analyzer produces this diagnostic when a map pattern has either one type argument or more than two type arguments. Map patterns can have either two type arguments or zero type arguments, but can't have any other number.

The following code produces this diagnostic because the map pattern (<int>{}) has one type argument:

Add or remove type arguments until there are two, or none:

Map literals require two type arguments or none, but {0} found.

The analyzer produces this diagnostic when a map literal has either one or more than two type arguments.

The following code produces this diagnostic because the map literal has three type arguments when it can have either two or zero:

Remove all except two of the type arguments:

The library '{0}' is internal and can't be exported.

The analyzer produces this diagnostic when it finds an export whose dart: URI references an internal library.

The following code produces this diagnostic because _interceptors is an internal library:

Remove the export directive.

The symbol '{0}' is defined in a legacy library, and can't be re-exported from a library with null safety enabled.

The analyzer produces this diagnostic when a library that was opted in to null safety exports another library, and the exported library is opted out of null safety.

Given a library that is opted out of null safety:

The following code produces this diagnostic because it's exporting symbols from an opted-out library:

If you're able to do so, migrate the exported library so that it doesn't need to opt out:

If you can't migrate the library, then remove the export:

If the exported library (the one that is opted out) itself exports an opted-in library, then it's valid for your library to indirectly export the symbols from the opted-in library. You can do so by adding a hide combinator to the export directive in your library that hides all of the names declared in the opted-out library.

The exported library '{0}' can't have a part-of directive.

The analyzer produces this diagnostic when an export directive references a part rather than a library.

Given a file part.dart containing

The following code produces this diagnostic because the file part.dart is a part, and only libraries can be exported:

Either remove the export directive, or change the URI to be the URI of the library containing the part.

Expressions can't be used in a map literal.

The analyzer produces this diagnostic when the analyzer finds an expression, rather than a map entry, in what appears to be a map literal.

The following code produces this diagnostic:

If the expression is intended to compute either a key or a value in an entry, fix the issue by replacing the expression with the key or the value. For example:

Classes can only extend other classes.

The analyzer produces this diagnostic when an extends clause contains a name that is declared to be something other than a class.

The following code produces this diagnostic because f is declared to be a function:

If you want the class to extend a class other than Object, then replace the name in the extends clause with the name of that class:

If you want the class to extend Object, then remove the extends clause:

Extension '{0}' can't be used as an expression.

The analyzer produces this diagnostic when the name of an extension is used in an expression other than in an extension override or to qualify an access to a static member of the extension. Because classes define a type, the name of a class can be used to refer to the instance of Type representing the type of the class. Extensions, on the other hand, don't define a type and can't be used as a type literal.

The following code produces this diagnostic because E is an extension:

Replace the name of the extension with a name that can be referenced, such as a static member defined on the extension:

An extension can't define static member '{0}' and an instance member with the same name.

The analyzer produces this diagnostic when an extension declaration contains both an instance member and a static member that have the same name. The instance member and the static member can't have the same name because it's unclear which member is being referenced by an unqualified use of the name within the body of the extension.

The following code produces this diagnostic because the name a is being used for two different members:

Rename or remove one of the members:

Extensions can't declare abstract members.

The analyzer produces this diagnostic when an abstract declaration is declared in an extension. Extensions can declare only concrete members.

The following code produces this diagnostic because the method a doesn't have a body:

Either provide an implementation for the member or remove it.

Extensions can't declare constructors.

The analyzer produces this diagnostic when a constructor declaration is found in an extension. It isn't valid to define a constructor because extensions aren't classes, and it isn't possible to create an instance of an extension.

The following code produces this diagnostic because there is a constructor declaration in E:

Remove the constructor or replace it with a static method.

Extensions can't declare instance fields.

The analyzer produces this diagnostic when an instance field declaration is found in an extension. It isn't valid to define an instance field because extensions can only add behavior, not state.

The following code produces this diagnostic because s is an instance field:

If the value can be computed without storing it in a field, then try using a getter or a method:

If the value must be stored, but is the same for every instance, try using a static field:

If each instance needs to have its own value stored, then try using a getter and setter pair backed by a static Expando:

Extensions can't declare members with the same name as a member declared by 'Object'.

The analyzer produces this diagnostic when an extension declaration declares a member with the same name as a member declared in the class Object. Such a member can never be used because the member in Object is always found first.

The following code produces this diagnostic because toString is defined by Object:

Remove the member or rename it so that the name doesn't conflict with the member in Object:

An extension override can't be used to access a static member from an extension.

The analyzer produces this diagnostic when an extension override is the receiver of the invocation of a static member. Similar to static members in classes, the static members of an extension should be accessed using the name of the extension, not an extension override.

The following code produces this diagnostic because m is static:

Replace the extension override with the name of the extension:

The type of the argument to the extension override '{0}' isn't assignable to the extended type '{1}'.

The analyzer produces this diagnostic when the argument to an extension override isn't assignable to the type being extended by the extension.

The following code produces this diagnostic because 3 isn't a String:

If you're using the correct extension, then update the argument to have the correct type:

If there's a different extension that's valid for the type of the argument, then either replace the name of the extension or unwrap the argument so that the correct extension is found.

An extension override can only be used to access instance members.

The analyzer produces this diagnostic when an extension override is found that isn't being used to access one of the members of the extension. The extension override syntax doesn't have any runtime semantics; it only controls which member is selected at compile time.

The following code produces this diagnostic because E(i) isn't an expression:

If you want to invoke one of the members of the extension, then add the invocation:

If you don't want to invoke a member, then unwrap the argument:

Extension overrides have no value so they can't be used as the receiver of a cascade expression.

The analyzer produces this diagnostic when an extension override is used as the receiver of a cascade expression. The value of a cascade expression e..m is the value of the receiver e, but extension overrides aren't expressions and don't have a value.

The following code produces this diagnostic because E(3) isn't an expression:

Use . rather than ..:

If there are multiple cascaded accesses, you'll need to duplicate the extension override for each one.

Extension type constructors can't declare super formal parameters.

The analyzer produces this diagnostic when a constructor in an extension type has a super parameter. Super parameters aren't valid because extension types don't have a superclass.

The following code produces this diagnostic because the named constructor n contains a super parameter:

If you need the parameter, replace the super parameter with a normal parameter:

If you don't need the parameter, remove the super parameter:

Extension type constructors can't include super initializers.

The analyzer produces this diagnostic when a constructor in an extension type includes an invocation of a super constructor in the initializer list. Because extension types don't have a superclass, there's no constructor to invoke.

The following code produces this diagnostic because the constructor E.n invokes a super constructor in its initializer list:

Remove the invocation of the super constructor:

Extension types can't declare instance fields.

The analyzer produces this diagnostic when there's a field declaration in the body of an extension type declaration.

The following code produces this diagnostic because the extension type E declares a field named f:

If you don't need the field, then remove it or replace it with a getter and/or setter:

If you need the field, then convert the extension type into a class:

Extension types can't declare members with the same name as a member declared by 'Object'.

The analyzer produces this diagnostic when the body of an extension type declaration contains a member with the same name as one of the members declared by Object.

The following code produces this diagnostic because the class Object already defines a member named hashCode:

If you need a member with the implemented semantics, then rename the member:

If you don't need a member with the implemented semantics, then remove the member:

Extension types can't implement '{0}'.

The analyzer produces this diagnostic when an extension type implements a type that it isn't allowed to implement.

The following code produces this diagnostic because extension types can't implement the type dynamic:

Remove the disallowed type from the implements clause:

The extension type can't implement itself.

The analyzer produces this diagnostic when an extension type implements itself, either directly or indirectly.

The following code produces this diagnostic because the extension type A directly implements itself:

The following code produces this diagnostic because the extension type A indirectly implements itself (through B):

Break the cycle by removing a type from the implements clause of at least one of the types involved in the cycle:

'{0}' is not a supertype of '{1}', the representation type.

The analyzer produces this diagnostic when an extension type implements a type that isn't a supertype of the representation type.

The following code produces this diagnostic because the extension type A implements String, but String isn't a supertype of the representation type int:

If the representation type is correct, then remove or replace the type in the implements clause:

If the representation type isn't correct, then replace it with the correct type:

'{0}', the representation type of '{1}', is not a supertype of '{2}', the representation type of '{3}'.

The analyzer produces this diagnostic when an extension type implements another extension type, and the representation type of the implemented extension type isn't a subtype of the representation type of the implementing extension type.

The following code produces this diagnostic because the extension type B implements A, but the representation type of A (num) isn't a subtype of the representation type of B (String):

Either change the representation types of the two extension types so that the representation type of the implemented type is a supertype of the representation type of the implementing type:

Or remove the implemented type from the implements clause:

The extension type '{0}' has more than one distinct member named '{1}' from implemented types.

The analyzer produces this diagnostic when an extension type implements two or more other types, and at least two of those types declare a member with the same name.

The following code produces this diagnostic because the extension type C implements both A and B, and both declare a member named m:

If the extension type doesn't need to implement all of the listed types, then remove all but one of the types introducing the conflicting members:

If the extension type needs to implement all of the listed types but you can rename the members in those types, then give the conflicting members unique names:

The extension type representation can't depend on itself.

The analyzer produces this diagnostic when an extension type has a representation type that depends on the extension type itself, either directly or indirectly.

The following code produces this diagnostic because the representation type of the extension type A depends on A directly:

The following two code examples produce this diagnostic because the representation type of the extension type A depends on A indirectly through the extension type B:

Remove the dependency by choosing a different representation type for at least one of the types in the cycle:

The representation type can't be a bottom type.

The analyzer produces this diagnostic when the representation type of an extension type is the bottom type Never. The type Never can't be the representation type of an extension type because there are no values that can be extended.

The following code produces this diagnostic because the representation type of the extension type E is Never:

Replace the extension type with a different type:

'{0}' must have a method body because '{1}' is an extension type.

The analyzer produces this diagnostic when an extension type declares an abstract member. Because extension type member references are resolved statically, an abstract member in an extension type could never be executed.

The following code produces this diagnostic because the method m in the extension type E is abstract:

If the member is intended to be executable, then provide an implementation of the member:

If the member isn't intended to be executable, then remove it:

External fields can't have initializers.

External variables can't have initializers.

The analyzer produces this diagnostic when a field or variable marked with the keyword external has an initializer, or when an external field is initialized in a constructor.

The following code produces this diagnostic because the external field x is assigned a value in an initializer:

The following code produces this diagnostic because the external field x has an initializer:

The following code produces this diagnostic because the external top level variable x has an initializer:

Remove the initializer:

Fields in a struct class must have exactly one annotation indicating the native type.

The analyzer produces this diagnostic when a field in a subclass of Struct has more than one annotation describing the native type of the field.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field x has two annotations describing the native type of the field:

Remove all but one of the annotations:

Too many positional arguments: {0} expected, but {1} found.

The analyzer produces this diagnostic when a method or function invocation has more positional arguments than the method or function allows.

The following code produces this diagnostic because f defines 2 parameters but is invoked with 3 arguments:

Remove the arguments that don't correspond to parameters:

Too many positional arguments: {0} expected, but {1} found.

The analyzer produces this diagnostic when a method or function invocation has more positional arguments than the method or function allows, but the method or function defines named parameters.

The following code produces this diagnostic because f defines 2 positional parameters but has a named parameter that could be used for the third argument:

If some of the arguments should be values for named parameters, then add the names before the arguments:

Otherwise, remove the arguments that don't correspond to positional parameters:

'Array's must have exactly one 'Array' annotation.

The analyzer produces this diagnostic when a field in a subclass of Struct has more than one annotation describing the size of the native array.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field a0 has two annotations that specify the size of the native array:

Remove all but one of the annotations:

There may be at most one @DefaultAsset annotation on a library.

The analyzer produces this diagnostic when a library directive has more than one DefaultAsset annotation associated with it.

The following code produces this diagnostic because the library directive has two DefaultAsset annotations associated with it:

Remove all but one of the DefaultAsset annotations:

Native functions and fields must have exactly one @Native annotation.

The analyzer produces this diagnostic when there is more than one Native annotation on a single declaration.

The following code produces this diagnostic because the function f has two Native annotations associated with it:

Remove all but one of the annotations:

Native functions must be declared external.

The analyzer produces this diagnostic when a function annotated as being @Native isn't marked as external.

The following code produces this diagnostic because the function free is annotated as being @Native, but the function isn't marked as external:

If the function is a native function, then add the modifier external before the return type:

Unexpected number of Native annotation parameters. Expected {0} but has {1}.

The analyzer produces this diagnostic when the number of parameters in the function type used as a type argument for the @Native annotation doesn't match the number of parameters in the function being annotated.

The following code produces this diagnostic because the function type used as a type argument for the @Native annotation (Void Function(Double)) has one argument and the type of the annotated function (void f(double, double)) has two arguments:

If the annotated function is correct, then update the function type in the @Native annotation to match:

If the function type in the @Native annotation is correct, then update the annotated function to match:

Unexpected number of Native annotation parameters. Expected {0} but has {1}. Native instance method annotation must have receiver as first argument.

The analyzer produces this diagnostic when the type argument used on the @Native annotation of a native method doesn't include a type for the receiver of the method.

The following code produces this diagnostic because the type argument on the @Native annotation (Void Function(Double)) doesn't include a type for the receiver of the method:

Add an initial parameter whose type is the same as the class in which the native method is being declared:

The field '{0}' can't be initialized twice in the same constructor.

The analyzer produces this diagnostic when the initializer list of a constructor initializes a field more than once. There is no value to allow both initializers because only the last value is preserved.

The following code produces this diagnostic because the field f is being initialized twice:

Remove one of the initializers:

Fields can't be initialized in the constructor if they are final and were already initialized at their declaration.

The analyzer produces this diagnostic when a final field is initialized in both the declaration of the field and in an initializer in a constructor. Final fields can only be assigned once, so it can't be initialized in both places.

The following code produces this diagnostic because f is :

If the initialization doesn't depend on any values passed to the constructor, and if all of the constructors need to initialize the field to the same value, then remove the initializer from the constructor:

If the initialization depends on a value passed to the constructor, or if different constructors need to initialize the field differently, then remove the initializer in the field's declaration:

Fields can't be initialized in both the parameter list and the initializers.

The analyzer produces this diagnostic when a field is initialized in both the parameter list and in the initializer list of a constructor.

The following code produces this diagnostic because the field f is initialized both by an initializing formal parameter and in the initializer list:

If the field should be initialized by the parameter, then remove the initialization in the initializer list:

If the field should be initialized in the initializer list and the parameter isn't needed, then remove the parameter:

If the field should be initialized in the initializer list and the parameter is needed, then make it a normal parameter:

Initializing formal parameters can't be used in factory constructors.

The analyzer produces this diagnostic when a factory constructor has an initializing formal parameter. Factory constructors can't assign values to fields because no instance is created; hence, there is no field to assign.

The following code produces this diagnostic because the factory constructor uses an initializing formal parameter:

Replace the initializing formal parameter with a normal parameter:

Constructors in subclasses of 'Struct' and 'Union' can't have field initializers.

The analyzer produces this diagnostic when a constructor in a subclass of either Struct or Union has one or more field initializers.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C has a constructor with an initializer for the field f:

Remove the field initializer:

The initializer type '{0}' can't be assigned to the field type '{1}' in a const constructor.

The initializer type '{0}' can't be assigned to the field type '{1}'.

The analyzer produces this diagnostic when the initializer list of a constructor initializes a field to a value that isn't assignable to the field.

The following code produces this diagnostic because 0 has the type int, and an int can't be assigned to a field of type String:

If the type of the field is correct, then change the value assigned to it so that the value has a valid type:

If the type of the value is correct, then change the type of the field to allow the assignment:

Field formal parameters can only be used in a constructor.

Initializing formal parameters can only be used in constructors.

The analyzer produces this diagnostic when an initializing formal parameter is used in the parameter list for anything other than a constructor.

The following code produces this diagnostic because the initializing formal parameter this.x is being used in the method m:

Replace the initializing formal parameter with a normal parameter and assign the field within the body of the method:

The redirecting constructor can't have a field initializer.

The analyzer produces this diagnostic when a redirecting constructor initializes a field in the object. This isn't allowed because the instance that has the field hasn't been created at the point at which it should be initialized.

The following code produces this diagnostic because the constructor C.zero, which redirects to the constructor C, has an initializing formal parameter that initializes the field f:

The following code produces this diagnostic because the constructor C.zero, which redirects to the constructor C, has an initializer that initializes the field f:

If the initialization is done by an initializing formal parameter, then use a normal parameter:

If the initialization is done in an initializer, then remove the initializer:

The parameter type '{0}' is incompatible with the field type '{1}'.

The analyzer produces this diagnostic when the type of an initializing formal parameter isn't assignable to the type of the field being initialized.

The following code produces this diagnostic because the initializing formal parameter has the type String, but the type of the field is int. The parameter must have a type that is a subtype of the field's type.

If the type of the field is incorrect, then change the type of the field to match the type of the parameter, and consider removing the type from the parameter:

If the type of the parameter is incorrect, then remove the type of the parameter:

If the types of both the field and the parameter are correct, then use an initializer rather than an initializing formal parameter to convert the parameter value into a value of the correct type:

Fields in subclasses of 'Struct' and 'Union' can't have initializers.

The analyzer produces this diagnostic when a field in a subclass of Struct has an initializer.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field p has an initializer:

Remove the initializer:

Fields of 'Struct' and 'Union' subclasses must be marked external.

The analyzer produces this diagnostic when a field in a subclass of either Struct or Union isn't marked as being external.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field a isn't marked as being external:

Add the required external modifier:

'{0}' is final and was given a value when it was declared, so it can't be set to a new value.

The analyzer produces this diagnostic when a final field is initialized twice: once where it's declared and once by a constructor's parameter.

The following code produces this diagnostic because the field f is initialized twice:

If the field should have the same value for all instances, then remove the initialization in the parameter list:

If the field can have different values in different instances, then remove the initialization in the declaration:

The final variable '{0}' must be initialized.

The analyzer produces this diagnostic when a final field or variable isn't initialized.

The following code produces this diagnostic because x doesn't have an initializer:

For variables and static fields, you can add an initializer:

For instance fields, you can add an initializer as shown in the previous example, or you can initialize the field in every constructor. You can initialize the field by using an initializing formal parameter:

You can also initialize the field by using an initializer in the constructor:

All final variables must be initialized, but '{0}' and '{1}' aren't.

All final variables must be initialized, but '{0}' isn't.

All final variables must be initialized, but '{0}', '{1}', and {2} others aren't.

The analyzer produces this diagnostic when a class defines one or more final instance fields without initializers and has at least one constructor that doesn't initialize those fields. All final instance fields must be initialized when the instance is created, either by the field's initializer or by the constructor.

The following code produces this diagnostic:

If the value should be passed in to the constructor directly, then use an initializing formal parameter to initialize the field value:

If the value should be computed indirectly from a value provided by the caller, then add a parameter and include an initializer:

If the value of the field doesn't depend on values that can be passed to the constructor, then add an initializer for the field as part of the field declaration:

If the value of the field doesn't depend on values that can be passed to the constructor but different constructors need to initialize it to different values, then add an initializer for the field in the initializer list:

However, if the value is the same for all instances, then consider using a static field instead of an instance field:

The value of the 'flutter' field is expected to be a map.

The analyzer produces this diagnostic when the value of the flutter key isn't a map.

The following code produces this diagnostic because the value of the top-level flutter key is a string:

If you need to specify Flutter-specific options, then change the value to be a map:

If you don't need to specify Flutter-specific options, then remove the flutter key:

The type '{0}' used in the 'for' loop must implement '{1}' with a type argument that can be assigned to '{2}'.

The analyzer produces this diagnostic when the Iterable or Stream in a for-in loop has an element type that can't be assigned to the loop variable.

The following code produces this diagnostic because <String>[] has an element type of String, and String can't be assigned to the type of e (int):

If the type of the loop variable is correct, then update the type of the iterable:

If the type of the iterable is correct, then update the type of the loop variable:

The type '{0}' used in the 'for' loop must implement '{1}'.

The analyzer produces this diagnostic when the expression following in in a for-in loop has a type that isn't a subclass of Iterable.

The following code produces this diagnostic because m is a Map, and Map isn't a subclass of Iterable:

Replace the expression with one that produces an iterable value:

A for-in loop variable can't be a 'const'.

The analyzer produces this diagnostic when the loop variable declared in a for-in loop is declared to be a const. The variable can't be a const because the value can't be computed at compile time.

The following code produces this diagnostic because the loop variable x is declared to be a const:

If there's a type annotation, then remove the const modifier from the declaration.

If there's no type, then replace the const modifier with final, var, or a type annotation:

A method tear-off on a receiver whose type is 'dynamic' can't have type arguments.

The analyzer produces this diagnostic when an instance method is being torn off from a receiver whose type is dynamic, and the tear-off includes type arguments. Because the analyzer can't know how many type parameters the method has, or whether it has any type parameters, there's no way it can validate that the type arguments are correct. As a result, the type arguments aren't allowed.

The following code produces this diagnostic because the type of p is dynamic and the tear-off of m has type arguments:

If you can use a more specific type than dynamic, then change the type of the receiver:

If you can't use a more specific type, then remove the type arguments:

The class '{0}' can't extend 'Struct' or 'Union' because '{0}' is generic.

The analyzer produces this diagnostic when a subclass of either Struct or Union has a type parameter.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class S defines the type parameter T:

Remove the type parameters from the class:

The return type of getter '{0}' is '{1}' which isn't a subtype of the type '{2}' of its setter '{3}'.

The analyzer produces this diagnostic when the return type of a getter isn't a subtype of the type of the parameter of a setter with the same name.

The subtype relationship is a requirement whether the getter and setter are in the same class or whether one of them is in a superclass of the other.

The following code produces this diagnostic because the return type of the getter x is num, the parameter type of the setter x is int, and num isn't a subtype of int:

If the type of the getter is correct, then change the type of the setter:

If the type of the setter is correct, then change the type of the getter:

Functions marked 'async*' must have a return type that is a supertype of 'Stream' for some type 'T'.

The analyzer produces this diagnostic when the body of a function has the async* modifier even though the return type of the function isn't either Stream or a supertype of Stream.

The following code produces this diagnostic because the body of the function f has the 'async*' modifier even though the return type int isn't a supertype of Stream:

If the function should be asynchronous, then change the return type to be either Stream or a supertype of Stream:

If the function should be synchronous, then remove the async* modifier:

Functions marked 'async' must have a return type which is a supertype of 'Future'.

The analyzer produces this diagnostic when the body of a function has the async modifier even though the return type of the function isn't assignable to Future.

The following code produces this diagnostic because the body of the function f has the async modifier even though the return type isn't assignable to Future:

If the function should be asynchronous, then change the return type to be assignable to Future:

If the function should be synchronous, then remove the async modifier:

A concrete instance member named '{0}' can't be declared in a class that implements 'Enum'.

A concrete instance member named '{0}' can't be inherited from '{1}' in a class that implements 'Enum'.

The analyzer produces this diagnostic when either an enum declaration, a class that implements Enum, or a mixin with a superclass constraint of Enum, declares or inherits a concrete instance member named either index, hashCode, or ==.

The following code produces this diagnostic because the enum E declares an instance getter named index:

The following code produces this diagnostic because the class C, which implements Enum, declares an instance field named hashCode:

The following code produces this diagnostic because the class C, which indirectly implements Enum through the class A, declares an instance getter named hashCode:

The following code produces this diagnostic because the mixin M, which has Enum in the on clause, declares an explicit operator named ==:

Rename the conflicting member:

An instance member named 'values' can't be declared in a class that implements 'Enum'.

An instance member named 'values' can't be inherited from '{0}' in a class that implements 'Enum'.

The analyzer produces this diagnostic when either a class that implements Enum or a mixin with a superclass constraint of Enum has an instance member named values.

The following code produces this diagnostic because the class C, which implements Enum, declares an instance field named values:

The following code produces this diagnostic because the class B, which implements Enum, inherits an instance method named values from A:

Change the name of the conflicting member:

Functions marked 'sync*' must have a return type that is a supertype of 'Iterable' for some type 'T'.

The analyzer produces this diagnostic when the body of a function has the sync* modifier even though the return type of the function isn't either Iterable or a supertype of Iterable.

The following code produces this diagnostic because the body of the function f has the 'sync*' modifier even though the return type int isn't a supertype of Iterable:

If the function should return an iterable, then change the return type to be either Iterable or a supertype of Iterable:

If the function should return a single value, then remove the sync* modifier:

Classes and mixins can only implement other classes and mixins.

The analyzer produces this diagnostic when a name used in the implements clause of a class or mixin declaration is defined to be something other than a class or mixin.

The following code produces this diagnostic because x is a variable rather than a class or mixin:

If the name is the name of an existing class or mixin that's already being imported, then add a prefix to the import so that the local definition of the name doesn't shadow the imported name.

If the name is the name of an existing class or mixin that isn't being imported, then add an import, with a prefix, for the library in which it's declared.

Otherwise, either replace the name in the implements clause with the name of an existing class or mixin, or remove the name from the implements clause.

'{0}' can only be implemented once.

The analyzer produces this diagnostic when a single class is specified more than once in an implements clause.

The following code produces this diagnostic because A is in the list twice:

Remove all except one occurrence of the class name:

'{0}' can't be used in both the 'extends' and 'implements' clauses.

'{0}' can't be used in both the 'extends' and 'with' clauses.

The analyzer produces this diagnostic when a class is listed in the extends clause of a class declaration and also in either the implements or with clause of the same declaration.

The following code produces this diagnostic because the class A is used in both the extends and implements clauses for the class B:

The following code produces this diagnostic because the class A is used in both the extends and with clauses for the class B:

If you want to inherit the implementation from the class, then remove the class from the implements clause:

If you don't want to inherit the implementation from the class, then remove the extends clause:

The implicitly invoked unnamed constructor from '{0}' has required parameters.

The analyzer produces this diagnostic when a constructor implicitly invokes the unnamed constructor from the superclass, the unnamed constructor of the superclass has a required parameter, and there's no super parameter corresponding to the required parameter.

The following code produces this diagnostic because the unnamed constructor in the class B implicitly invokes the unnamed constructor in the class A, but the constructor in A has a required positional parameter named x:

The following code produces this diagnostic because the unnamed constructor in the class B implicitly invokes the unnamed constructor in the class A, but the constructor in A has a required named parameter named x:

If you can add a parameter to the constructor in the subclass, then add a super parameter corresponding to the required parameter in the superclass' constructor. The new parameter can either be required:

or it can be optional:

If you can't add a parameter to the constructor in the subclass, then add an explicit super constructor invocation with the required argument:

The instance member '{0}' can't be accessed in an initializer.

The analyzer produces this diagnostic when it finds a reference to an instance member in a constructor's initializer list.

The following code produces this diagnostic because defaultX is an instance member:

If the member can be made static, then do so:

If not, then replace the reference in the initializer with a different expression that doesn't use an instance member:

The imported library defines a top-level function named 'loadLibrary' that is hidden by deferring this library.

The analyzer produces this diagnostic when a library that declares a function named loadLibrary is imported using a deferred import. A deferred import introduces an implicit function named loadLibrary. This function is used to load the contents of the deferred library, and the implicit function hides the explicit declaration in the deferred library.

For more information, check out Lazily loading a library.

Given a file a.dart that defines a function named loadLibrary:

The following code produces this diagnostic because the implicit declaration of a.loadLibrary is hiding the explicit declaration of loadLibrary in a.dart:

If the imported library isn't required to be deferred, then remove the keyword deferred:

If the imported library is required to be deferred and you need to reference the imported function, then rename the function in the imported library:

If the imported library is required to be deferred and you don't need to reference the imported function, then add a hide clause:

The library '{0}' is internal and can't be imported.

The analyzer produces this diagnostic when it finds an import whose dart: URI references an internal library.

The following code produces this diagnostic because _interceptors is an internal library:

Remove the import directive.

The library '{0}' is legacy, and shouldn't be imported into a null safe library.

The analyzer produces this diagnostic when a library that is null safe imports a library that isn't null safe.

Given a file a.dart that contains the following:

The following code produces this diagnostic because a library that null safe is importing a library that isn't null safe:

If you can migrate the imported library to be null safe, then migrate it and update or remove the migrated library's language version.

If you can't migrate the imported library, then the importing library needs to have a language version that is before 2.12, when null safety was enabled by default.

The imported library '{0}' can't have a part-of directive.

The analyzer produces this diagnostic when a part file is imported into a library.

Given a part file named part.dart containing the following:

The following code produces this diagnostic because imported files can't have a part-of directive:

Import the library that contains the part file rather than the part file itself.

Superinterfaces don't have a valid override for '{0}': {1}.

The analyzer produces this diagnostic when a class inherits two or more conflicting signatures for a member and doesn't provide an implementation that satisfies all the inherited signatures.

The following code produces this diagnostic because C is inheriting the declaration of m from A, and that implementation isn't consistent with the signature of m that's inherited from B:

Add an implementation of the method that satisfies all the inherited signatures:

Parts must have exactly the same language version override as the library.

The analyzer produces this diagnostic when a part file has a language version override comment that specifies a different language version than the one being used for the library to which the part belongs.

Given a part file named part.dart that contains the following:

The following code produces this diagnostic because the parts of a library must have the same language version as the defining compilation unit:

Remove the language version override from the part file, so that it implicitly uses the same version as the defining compilation unit:

If necessary, either adjust the language version override in the defining compilation unit to be appropriate for the code in the part, or migrate the code in the part file to be consistent with the new language version.

The variable '{0}' has a different type and/or finality in this branch of the logical-or pattern.

The analyzer produces this diagnostic when a pattern variable that is declared on all branches of a logical-or pattern doesn't have the same type on every branch. It is also produced when the variable has a different finality on different branches. A pattern variable declared on multiple branches of a logical-or pattern is required to have the same type and finality in each branch, so that the type and finality of the variable can be known in code that's guarded by the logical-or pattern.

The following code produces this diagnostic because the variable a is defined to be an int on one branch and a double on the other:

The following code produces this diagnostic because the variable a is final in the first branch and isn't final in the second branch:

If the finality of the variable is different, decide whether it should be final or not final and make the cases consistent:

If the type of the variable is different and the type isn't critical to the condition being matched, then ensure that the variable has the same type on both branches:

If the type of the variable is different and the type is critical to the condition being matched, then consider breaking the condition into multiple if statements or case clauses:

'{0}' isn't a field in the enclosing class.

The analyzer produces this diagnostic when a constructor initializes a field that isn't declared in the class containing the constructor. Constructors can't initialize fields that aren't declared and fields that are inherited from superclasses.

The following code produces this diagnostic because the initializer is initializing x, but x isn't a field in the class:

If a different field should be initialized, then change the name to the name of the field:

If the field must be declared, then add a declaration:

'{0}' is a static field in the enclosing class. Fields initialized in a constructor can't be static.

The analyzer produces this diagnostic when a static field is initialized in a constructor using either an initializing formal parameter or an assignment in the initializer list.

The following code produces this diagnostic because the static field a is being initialized by the initializing formal parameter this.a:

If the field should be an instance field, then remove the keyword static:

If you intended to initialize an instance field and typed the wrong name, then correct the name of the field being initialized:

If you really want to initialize the static field, then move the initialization into the constructor body:

'{0}' isn't a field in the enclosing class.

The analyzer produces this diagnostic when an initializing formal parameter is found in a constructor in a class that doesn't declare the field being initialized. Constructors can't initialize fields that aren't declared and fields that are inherited from superclasses.

The following code produces this diagnostic because the field x isn't defined:

If the field name was wrong, then change it to the name of an existing field:

If the field name is correct but hasn't yet been defined, then declare the field:

If the parameter is needed but shouldn't initialize a field, then convert it to a normal parameter and use it:

If the parameter isn't needed, then remove it:

The static {1} '{0}' can't be accessed through an instance.

The analyzer produces this diagnostic when an access operator is used to access a static member through an instance of the class.

The following code produces this diagnostic because zero is a static field, but it's being accessed as if it were an instance field:

Use the class to access the static member:

Instance members can't be accessed from a factory constructor.

The analyzer produces this diagnostic when a factory constructor contains an unqualified reference to an instance member. In a generative constructor, the instance of the class is created and initialized before the body of the constructor is executed, so the instance can be bound to this and accessed just like it would be in an instance method. But, in a factory constructor, the instance isn't created before executing the body, so this can't be used to reference it.

The following code produces this diagnostic because x isn't in scope in the factory constructor:

Rewrite the code so that it doesn't reference the instance member:

Instance members can't be accessed from a static method.

The analyzer produces this diagnostic when a static method contains an unqualified reference to an instance member.

The following code produces this diagnostic because the instance field x is being referenced in a static method:

If the method must reference the instance member, then it can't be static, so remove the keyword:

If the method can't be made an instance method, then add a parameter so that an instance of the class can be passed in:

Abstract classes can't be instantiated.

The analyzer produces this diagnostic when it finds a constructor invocation and the constructor is declared in an abstract class. Even though you can't create an instance of an abstract class, abstract classes can declare constructors that can be invoked by subclasses.

The following code produces this diagnostic because C is an abstract class:

If there's a concrete subclass of the abstract class that can be used, then create an instance of the concrete subclass.

Enums can't be instantiated.

The analyzer produces this diagnostic when an enum is instantiated. It's invalid to create an instance of an enum by invoking a constructor; only the instances named in the declaration of the enum can exist.

The following code produces this diagnostic because the enum E is being instantiated:

If you intend to use an instance of the enum, then reference one of the constants defined in the enum:

If you intend to use an instance of a class, then use the name of that class in place of the name of the enum.

Type aliases that expand to a type parameter can't be instantiated.

The analyzer produces this diagnostic when a constructor invocation is found where the type being instantiated is a type alias for one of the type parameters of the type alias. This isn't allowed because the value of the type parameter is a type rather than a class.

The following code produces this diagnostic because it creates an instance of A, even though A is a type alias that is defined to be equivalent to a type parameter:

Use either a class name or a type alias defined to be a class, rather than a type alias defined to be a type parameter:

The integer literal is being used as a double, but can't be represented as a 64-bit double without overflow or loss of precision: '{0}'.

The analyzer produces this diagnostic when an integer literal is being implicitly converted to a double, but can't be represented as a 64-bit double without overflow or loss of precision. Integer literals are implicitly converted to a double if the context requires the type double.

The following code produces this diagnostic because the integer value 9223372036854775807 can't be represented exactly as a double:

If you need to use the exact value, then use the class BigInt to represent the value:

If you need to use a double, then change the value to one that can be represented exactly:

The integer literal {0} can't be represented in 64 bits.

The analyzer produces this diagnostic when an integer literal has a value that is too large (positive) or too small (negative) to be represented in a 64-bit word.

The following code produces this diagnostic because the value can't be represented in 64 bits:

If you need to represent the current value, then wrap it in an instance of the class BigInt:

Annotation must be either a const variable reference or const constructor invocation.

The analyzer produces this diagnostic when an annotation is found that is using something that is neither a variable marked as const or the invocation of a const constructor.

Getters can't be used as annotations.

The following code produces this diagnostic because the variable v isn't a const variable:

The following code produces this diagnostic because f isn't a variable:

The following code produces this diagnostic because f isn't a constructor:

The following code produces this diagnostic because g is a getter:

If the annotation is referencing a variable that isn't a const constructor, add the keyword const to the variable's declaration:

If the annotation isn't referencing a variable, then remove it:

Constant values from a deferred library can't be used in annotations.

The analyzer produces this diagnostic when a constant defined in a library that is imported as a deferred library is referenced in the argument list of an annotation. Annotations are evaluated at compile time, and values from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because the constant pi is being referenced in the argument list of an annotation, even though the library that defines it is being imported as a deferred library:

If you need to reference the imported constant, then remove the deferred keyword:

If the import is required to be deferred and there's another constant that is appropriate, then use that constant in place of the constant from the deferred library.

Constant values from a deferred library can't be used as annotations.

The analyzer produces this diagnostic when a constant from a library that is imported using a deferred import is used as an annotation. Annotations are evaluated at compile time, and constants from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because the constant pi is being used as an annotation when the library dart:math is imported as deferred:

If you need to reference the constant as an annotation, then remove the keyword deferred from the import:

If you can use a different constant as an annotation, then replace the annotation with a different constant:

The annotation '{0}' can only be used on {1}.

The analyzer produces this diagnostic when an annotation is applied to a kind of declaration that it doesn't support.

The following code produces this diagnostic because the optionalTypeArgs annotation isn't defined to be valid for top-level variables:

Remove the annotation from the declaration.

A value of type '{0}' can't be assigned to a variable of type '{1}'.

The analyzer produces this diagnostic when the static type of an expression that is assigned to a variable isn't assignable to the type of the variable.

The following code produces this diagnostic because the type of the initializer (int) isn't assignable to the type of the variable (String):

If the value being assigned is always assignable at runtime, even though the static types don't reflect that, then add an explicit cast.

Otherwise, change the value being assigned so that it has the expected type. In the previous example, this might look like:

If you can't change the value, then change the type of the variable to be compatible with the type of the value being assigned:

Publishable packages can't have '{0}' dependencies.

The analyzer produces this diagnostic when a publishable package includes a package in the dependencies list of its pubspec.yaml file that isn't a pub-hosted dependency.

To learn more about the different types of dependency sources, check out Package dependencies.

The following code produces this diagnostic because the dependency on the package transmogrify isn't a pub-hosted dependency.

If you want to publish the package to pub.dev, then change the dependency to a hosted package that is published on pub.dev.

If the package isn't intended to be published on pub.dev, then add a publish_to: none entry to its pubspec.yaml file to mark it as not intended to be published:

The method {0} can't have an exceptional return value (the second argument) when the return type of the function is either 'void', 'Handle' or 'Pointer'.

The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction or NativeCallable.isolateLocal has a second argument (the exceptional return value) and the type to be returned from the invocation is either void, Handle or Pointer.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because a second argument is provided when the return type of f is void:

Remove the exception value:

The member '{0}' can't be exported as a part of a package's public API.

The analyzer produces this diagnostic when a public library exports a declaration that is marked with the internal annotation.

Given a file a.dart in the src directory that contains:

The following code, when found in a public library produces this diagnostic because the export directive is exporting a name that is only intended to be used internally:

If the export is needed, then add a hide clause to hide the internal names:

If the export isn't needed, then remove it.

The member '{0}' can't be exported as a part of a package's public API, but is indirectly exported as part of the signature of '{1}'.

The analyzer produces this diagnostic when a public library exports a top-level function with a return type or at least one parameter type that is marked with the internal annotation.

Given a file a.dart in the src directory that contains the following:

The following code produces this diagnostic because the function f has a parameter of type IntFunction, and IntFunction is only intended to be used internally:

If the function must be public, then make all the types in the function's signature public types.

If the function doesn't need to be exported, then stop exporting it, either by removing it from the show clause, adding it to the hide clause, or by removing the export.

Extension overrides must have exactly one argument: the value of 'this' in the extension method.

The analyzer produces this diagnostic when an extension override doesn't have exactly one argument. The argument is the expression used to compute the value of this within the extension method, so there must be one argument.

The following code produces this diagnostic because there are no arguments:

And, the following code produces this diagnostic because there's more than one argument:

Provide one argument for the extension override:

Factory method '{0}' must have a return type.

The analyzer produces this diagnostic when a method that is annotated with the factory annotation has a return type of void.

The following code produces this diagnostic because the method createC is annotated with the factory annotation but doesn't return any value:

Change the return type to something other than void:

Factory method '{0}' doesn't return a newly allocated object.

The analyzer produces this diagnostic when a method that is annotated with the factory annotation doesn't return a newly allocated object.

The following code produces this diagnostic because the method createC returns the value of a field rather than a newly created instance of C:

Change the method to return a newly created instance of the return type:

The name of a factory constructor must be the same as the name of the immediately enclosing class.

The analyzer produces this diagnostic when the name of a factory constructor isn't the same as the name of the surrounding class.

The following code produces this diagnostic because the name of the factory constructor (A) isn't the same as the surrounding class (C):

If the factory returns an instance of the surrounding class, and you intend it to be an unnamed factory constructor, then rename the factory:

If the factory returns an instance of the surrounding class, and you intend it to be a named factory constructor, then prefix the name of the factory constructor with the name of the surrounding class:

If the factory returns an instance of a different class, then move the factory to that class:

If the factory returns an instance of a different class, but you can't modify that class or don't want to move the factory, then convert it to be a static method:

Record field names can't be a dollar sign followed by an integer when the integer is the index of a positional field.

Record field names can't be private.

Record field names can't be the same as a member from 'Object'.

The analyzer produces this diagnostic when either a record literal or a record type annotation has a field whose name is invalid. The name is invalid if it is:

• private (starts with _)
• the same as one of the members defined on Object
• the same as the name of a positional field (an exception is made if the field is a positional field with the specified name)

The following code produces this diagnostic because the record literal has a field named toString, which is a method defined on Object:

The following code produces this diagnostic because the record type annotation has a field named hashCode, which is a getter defined on Object:

The following code produces this diagnostic because the record literal has a private field named _a:

The following code produces this diagnostic because the record type annotation has a private field named _a:

The following code produces this diagnostic because the record literal has a field named $1, which is also the name of a different positional parameter:

The following code produces this diagnostic because the record type annotation has a field named $1, which is also the name of a different positional parameter:

Rename the field:

Fields in struct classes can't have the type '{0}'. They can only be declared as 'int', 'double', 'Array', 'Pointer', or subtype of 'Struct' or 'Union'.

The analyzer produces this diagnostic when a field in a subclass of Struct has a type other than int, double, Array, Pointer, or subtype of Struct or Union.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field str has the type String, which isn't one of the allowed types for fields in a subclass of Struct:

Use one of the allowed types for the field:

'{1}.{0}' ('{2}') isn't a valid concrete implementation of '{3}.{0}' ('{4}').

The setter '{1}.{0}' ('{2}') isn't a valid concrete implementation of '{3}.{0}' ('{4}').

The analyzer produces this diagnostic when all of the following are true:

• A class defines an abstract member.
• There is a concrete implementation of that member in a superclass.
• The concrete implementation isn't a valid implementation of the abstract method.

The concrete implementation can be invalid because of incompatibilities in either the return type, the types of the method's parameters, or the type parameters.

The following code produces this diagnostic because the method A.add has a parameter of type int, and the overriding method B.add has a corresponding parameter of type num:

This is a problem because in an invocation of B.add like the following:

B.add is expecting to be able to take, for example, a double, but when the method A.add is executed (because it's the only concrete implementation of add), a runtime exception will be thrown because a double can't be assigned to a parameter of type int.

If the method in the subclass can conform to the implementation in the superclass, then change the declaration in the subclass (or remove it if it's the same):

If the method in the superclass can be generalized to be a valid implementation of the method in the subclass, then change the superclass method:

If neither the method in the superclass nor the method in the subclass can be changed, then provide a concrete implementation of the method in the subclass:

Inline function types can't be used for parameters in a generic function type.

The analyzer produces this diagnostic when a generic function type has a function-valued parameter that is written using the older inline function type syntax.

The following code produces this diagnostic because the parameter f, in the generic function type used to define F, uses the inline function type syntax:

Use the generic function syntax for the parameter's type:

Only public elements in a package's private API can be annotated as being internal.

The analyzer produces this diagnostic when a declaration is annotated with the internal annotation and that declaration is either in a public library or has a private name.

The following code, when in a public library, produces this diagnostic because the internal annotation can't be applied to declarations in a public library:

The following code, whether in a public or internal library, produces this diagnostic because the internal annotation can't be applied to declarations with private names:

If the declaration has a private name, then remove the annotation:

If the declaration has a public name and is intended to be internal to the package, then move the annotated declaration into an internal library (in other words, a library inside the src directory).

Otherwise, remove the use of the annotation:

The Dart language version override comment can't be followed by any non-whitespace characters.

The Dart language version override comment must be specified with a version number, like '2.0', after the '=' character.

The Dart language version override comment must be specified with an '=' character.

The Dart language version override comment must be specified with exactly two slashes.

The Dart language version override comment must be specified with the word 'dart' in all lower case.

The Dart language version override number can't be prefixed with a letter.

The Dart language version override number must begin with '@dart'.

The language version override can't specify a version greater than the latest known language version: {0}.{1}.

The language version override must be specified before any declaration or directive.

The analyzer produces this diagnostic when a comment that appears to be an attempt to specify a language version override doesn't conform to the requirements for such a comment. For more information, see Per-library language version selection.

The following code produces this diagnostic because the word dart must be lowercase in such a comment and because there's no equal sign between the word dart and the version number:

If the comment is intended to be a language version override, then change the comment to follow the correct format:

Only const constructors can have the @literal annotation.

The analyzer produces this diagnostic when the literal annotation is applied to anything other than a const constructor.

The following code produces this diagnostic because the constructor isn't a const constructor:

The following code produces this diagnostic because x isn't a constructor:

If the annotation is on a constructor and the constructor should always be invoked with const, when possible, then mark the constructor with the const keyword:

If the constructor can't be marked as const, then remove the annotation.

If the annotation is on anything other than a constructor, then remove the annotation:

The modifier '{0}' can't be applied to the body of a constructor.

The analyzer produces this diagnostic when the body of a constructor is prefixed by one of the following modifiers: async, async*, or sync*. Constructor bodies must be synchronous.

The following code produces this diagnostic because the body of the constructor for C is marked as being async:

If the constructor can be synchronous, then remove the modifier:

If the constructor can't be synchronous, then use a static method to create the instance instead:

Setters can't use 'async', 'async*', or 'sync*'.

The analyzer produces this diagnostic when the body of a setter is prefixed by one of the following modifiers: async, async*, or sync*. Setter bodies must be synchronous.

The following code produces this diagnostic because the body of the setter x is marked as being async:

If the setter can be synchronous, then remove the modifier:

If the setter can't be synchronous, then use a method to set the value instead:

The annotation '@nonVirtual' can only be applied to a concrete instance member.

The analyzer produces this diagnostic when the nonVirtual annotation is found on a declaration other than a member of a class, mixin, or enum, or if the member isn't a concrete instance member.

The following code produces this diagnostic because the annotation is on a class declaration rather than a member inside the class:

The following code produces this diagnostic because the method m is an abstract method:

The following code produces this diagnostic because the method m is a static method:

If the declaration isn't a member of a class, mixin, or enum, then remove the annotation:

If the member is intended to be a concrete instance member, then make it so:

If the member is not intended to be a concrete instance member, then remove the annotation:

The element can't be null, so the null-aware operator '?' is unnecessary.

The map entry key can't be null, so the null-aware operator '?' is unnecessary.

The map entry value can't be null, so the null-aware operator '?' is unnecessary.

The receiver can't be 'null' because of short-circuiting, so the null-aware operator '{0}' can't be used.

The receiver can't be null, so the null-aware operator '{0}' is unnecessary.

The analyzer produces this diagnostic when a null-aware operator (?., ?.., ?[, ?..[, or ...?) is used on a receiver that's known to be non-nullable.

The following code produces this diagnostic because s can't be null:

The following code produces this diagnostic because a can't be null:

The following code produces this diagnostic because s?.length can't return null:

The reason s?.length can't return null is because the null-aware operator following s short-circuits the evaluation of both length and isEven if s is null. In other words, if s is null, then neither length nor isEven will be invoked, and if s is non-null, then length can't return a null value. Either way, isEven can't be invoked on a null value, so the null-aware operator isn't necessary. See Understanding null safety for more details.

The following code produces this diagnostic because s can't be null.

The reason s can't be null, despite the fact that o can be null, is because of the cast to String, which is a non-nullable type. If o ever has the value null, the cast will fail and the invocation of length will not happen.

The following code produces this diagnostic because s can't be null:

Replace the null-aware operator with a non-null-aware equivalent; for example, change ?. to .:

(Note that the return type was also changed to be non-nullable, which might not be appropriate in some cases.)

'{1}.{0}' ('{2}') isn't a valid override of '{3}.{0}' ('{4}').

The setter '{1}.{0}' ('{2}') isn't a valid override of '{3}.{0}' ('{4}').

The analyzer produces this diagnostic when a member of a class is found that overrides a member from a supertype and the override isn't valid. An override is valid if all of these are true:

• It allows all of the arguments allowed by the overridden member.
• It doesn't require any arguments that aren't required by the overridden member.
• The type of every parameter of the overridden member is assignable to the corresponding parameter of the override.
• The return type of the override is assignable to the return type of the overridden member.

The following code produces this diagnostic because the type of the parameter s (String) isn't assignable to the type of the parameter i (int):

If the invalid method is intended to override the method from the superclass, then change it to conform:

If it isn't intended to override the method from the superclass, then rename it:

The member '{0}' is declared non-virtual in '{1}' and can't be overridden in subclasses.

The analyzer produces this diagnostic when a member of a class, mixin, or enum overrides a member that has the @nonVirtual annotation on it.

The following code produces this diagnostic because the method m in B overrides the method m in A, and the method m in A is annotated with the @nonVirtual annotation:

If the annotation on the method in the superclass is correct (the method in the superclass is not intended to be overridden), then remove or rename the overriding method:

If the method in the superclass is intended to be overridden, then remove the @nonVirtual annotation:

The variable '{0}' doesn't have the same type and/or finality in all cases that share this body.

The variable '{0}' is available in some, but not all cases that share this body.

The variable '{0}' is not available because there is a label or 'default' case.

The analyzer produces this diagnostic when multiple case clauses in a switch statement share a body, and at least one of them declares a variable that is referenced in the shared statements, but the variable is either not declared in all of the case clauses or it is declared in inconsistent ways.

If the variable isn't declared in all of the case clauses, then it won't have a value if one of the clauses that doesn't declare the variable is the one that matches and executes the body. This includes the situation where one of the case clauses is the default clause.

If the variable is declared in inconsistent ways, either being final in some cases and not final in others or having a different type in different cases, then the semantics of what the type or finality of the variable should be are not defined.

The following code produces this diagnostic because the variable a is only declared in one of the case clauses, and won't have a value if the second clause is the one that matched x:

The following code produces this diagnostic because the variable a isn't declared in the default clause, and won't have a value if the body is executed because none of the other clauses matched x:

The following code produces this diagnostic because the variable a won't have a value if the body is executed because a different group of cases caused control to continue at the label:

The following code produces this diagnostic because the variable a, while being assigned in all of the case clauses, doesn't have then same type associated with it in every clause:

The following code produces this diagnostic because the variable a is final in the first case clause and isn't final in the second case clause:

If the variable isn't declared in all of the cases, and you need to reference it in the statements, then declare it in the other cases:

If the variable isn't declared in all of the cases, and you don't need to reference it in the statements, then remove the references to it and remove the declarations from the other cases:

If the type of the variable is different, decide the type the variable should have and make the cases consistent:

If the finality of the variable is different, decide whether it should be final or not final and make the cases consistent:

The 'platforms' field must be a map with platforms as keys.

The analyzer produces this diagnostic when a top-level platforms field is specified, but its value is not a map with keys. To learn more about specifying your package's supported platforms, check out the documentation on platform declarations.

The following pubspec.yaml produces this diagnostic because platforms should be a map.

If you can rely on automatic platform detection, then omit the top-level platforms field.

If you need to manually specify the list of supported platforms, then write the platforms field as a map with platform names as keys.

Generative enum constructors can only be used as targets of redirection.

The analyzer produces this diagnostic when a generative constructor defined on an enum is used anywhere other than to create one of the enum constants or as the target of a redirection from another constructor in the same enum.

The following code produces this diagnostic because the constructor for E is being used to create an instance in the function f:

If there's an enum value with the same value, or if you add such a constant, then reference the constant directly:

If you need to use a constructor invocation, then use a factory constructor:

Invalid reference to 'this' expression.

The analyzer produces this diagnostic when this is used outside of an instance method or a generative constructor. The reserved word this is only defined in the context of an instance method, a generative constructor, or the initializer of a late instance field declaration.

The following code produces this diagnostic because v is a top-level variable:

Use a variable of the appropriate type in place of this, declaring it if necessary:

A value of type '{0}' can't be returned by the 'onError' handler because it must be assignable to '{1}'.

The return type '{0}' isn't assignable to '{1}', as required by 'Future.catchError'.

The analyzer produces this diagnostic when an invocation of Future.catchError has an argument whose return type isn't compatible with the type returned by the instance of Future. At runtime, the method catchError attempts to return the value from the callback as the result of the future, which results in another exception being thrown.

The following code produces this diagnostic because future is declared to return an int while callback is declared to return a String, and String isn't a subtype of int:

The following code produces this diagnostic because the closure being passed to catchError returns an int while future is declared to return a String:

If the instance of Future is declared correctly, then change the callback to match:

If the declaration of the instance of Future is wrong, then change it to match the callback:

The annotation '@sealed' can only be applied to classes.

The analyzer produces this diagnostic when a declaration other than a class declaration has the @sealed annotation on it.

The following code produces this diagnostic because the @sealed annotation is on a method declaration:

Remove the annotation:

Super parameters can only be used in non-redirecting generative constructors.

The analyzer produces this diagnostic when a super parameter is used anywhere other than a non-redirecting generative constructor.

The following code produces this diagnostic because the super parameter x is in a redirecting generative constructor:

The following code produces this diagnostic because the super parameter x isn't in a generative constructor:

The following code produces this diagnostic because the super parameter x is in a method:

If the function containing the super parameter can be changed to be a non-redirecting generative constructor, then do so:

If the function containing the super parameter can't be changed to be a non-redirecting generative constructor, then remove the super:

Constant list literals can't use a type parameter in a type argument, such as '{0}'.

Constant map literals can't use a type parameter in a type argument, such as '{0}'.

Constant set literals can't use a type parameter in a type argument, such as '{0}'.

The analyzer produces this diagnostic when a type parameter is used in a type argument in a list, map, or set literal that is prefixed by const. This isn't allowed because the value of the type parameter (the actual type that will be used at runtime) can't be known at compile time.

The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant list:

The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant map:

The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant set:

If the type that will be used for the type parameter can be known at compile time, then remove the type parameter:

If the type that will be used for the type parameter can't be known until runtime, then remove the keyword const:

Invalid URI syntax: '{0}'.

The analyzer produces this diagnostic when a URI in a directive doesn't conform to the syntax of a valid URI.

The following code produces this diagnostic because '#' isn't a valid URI:

Replace the invalid URI with a valid URI.

Can't have modifier '{0}' in an extension.

The analyzer produces this diagnostic when a member declared inside an extension uses the keyword covariant in the declaration of a parameter. Extensions aren't classes and don't have subclasses, so the keyword serves no purpose.

The following code produces this diagnostic because i is marked as being covariant:

Remove the covariant keyword:

The member '{0}' can only be used within its package.

The analyzer produces this diagnostic when a reference to a declaration that is annotated with the internal annotation is found outside the package containing the declaration.

Given a package p that defines a library containing a declaration marked with the internal annotation:

The following code produces this diagnostic because it's referencing the class C, which isn't intended to be used outside the package p:

Remove the reference to the internal declaration.

An expression whose value is always 'null' can't be dereferenced.

The analyzer produces this diagnostic when an expression whose value will always be null is dereferenced.

The following code produces this diagnostic because x will always be null:

If the value is allowed to be something other than null, then change the type of the expression:

The class '{0}' can't be extended outside of its library because it's a final class.

The class '{0}' can't be extended outside of its library because it's an interface class.

The class '{0}' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.

The class '{0}' can't be implemented outside of its library because it's a base class.

The class '{0}' can't be implemented outside of its library because it's a final class.

The class '{0}' can't be used as a mixin superclass constraint outside of its library because it's a final class.

The mixin '{0}' can't be implemented outside of its library because it's a base mixin.

The analyzer produces this diagnostic when an extends, implements, with, or on clause uses a class or mixin in a way that isn't allowed given the modifiers on that class or mixin's declaration.

The message specifies how the declaration is being used and why it isn't allowed.

Given a file a.dart that defines a base class A:

The following code produces this diagnostic because the class B implements the class A, but the base modifier prevents A from being implemented outside of the library where it's defined:

Use of this type is restricted outside of its declaring library. If a different, unrestricted type is available that can provide similar functionality, then replace the type:

If there isn't a different type that would be appropriate, then remove the type, and possibly the whole clause:

The member '{0}' can only be used for overriding.

The analyzer produces this diagnostic when an instance member that is annotated with visibleForOverriding is referenced outside the library in which it's declared for any reason other than to override it.

Given a file a.dart containing the following declaration:

The following code produces this diagnostic because the method m is being invoked even though the only reason it's public is to allow it to be overridden:

Remove the invalid use of the member.

The member '{0}' can only be used within '{1}' or a test.

The analyzer produces this diagnostic when a member annotated with @visibleForTesting is referenced anywhere other than the library in which it is declared or in a library in the test directory.

Given a file c.dart that contains the following:

The following code, when not inside the test directory, produces this diagnostic because the method m is marked as being visible only for tests:

If the annotated member should not be referenced outside of tests, then remove the reference:

If it's OK to reference the annotated member outside of tests, then remove the annotation:

The member '{0}' is annotated with '{1}', but this annotation is only meaningful on declarations of public members.

The analyzer produces this diagnostic when either the visibleForTemplate or visibleForTesting annotation is applied to a non-public declaration.

The following code produces this diagnostic:

If the declaration doesn't need to be used by test code, then remove the annotation:

If it does, then make it public:

The annotation 'visibleForOverriding' can only be applied to a public instance member that can be overridden.

The analyzer produces this diagnostic when anything other than a public instance member of a class is annotated with visibleForOverriding. Because only public instance members can be overridden outside the defining library, there's no value to annotating any other declarations.

The following code produces this diagnostic because the annotation is on a class, and classes can't be overridden:

Remove the annotation:

The annotation 'visibleOutsideTemplate' can only be applied to a member of a class, enum, or mixin that is annotated with 'visibleForTemplate'.

The analyzer produces this diagnostic when the @visibleOutsideTemplate annotation is used incorrectly. This annotation is only meant to annotate members of a class, enum, or mixin that has the @visibleForTemplate annotation, to opt those members out of the visibility restrictions that @visibleForTemplate imposes.

The following code produces this diagnostic because there is no @visibleForTemplate annotation at the class level:

The following code produces this diagnostic because the annotation is on a class declaration, not a member of a class, enum, or mixin:

If the class is only visible so that templates can reference it, then add the @visibleForTemplate annotation to the class:

If the @visibleOutsideTemplate annotation is on anything other than a member of a class, enum, or mixin with the @visibleForTemplate annotation, remove the annotation:

The extension '{0}' doesn't define a 'call' method so the override can't be used in an invocation.

The analyzer produces this diagnostic when an extension override is used to invoke a function but the extension doesn't declare a call method.

The following code produces this diagnostic because the extension E doesn't define a call method:

If the extension is intended to define a call method, then declare it:

If the extended type defines a call method, then remove the extension override.

If the call method isn't defined, then rewrite the code so that it doesn't invoke the call method.

'{0}' isn't a function.

The analyzer produces this diagnostic when it finds a function invocation, but the name of the function being invoked is defined to be something other than a function.

The following code produces this diagnostic because Binary is the name of a function type, not a function:

Replace the name with the name of a function.

The expression doesn't evaluate to a function, so it can't be invoked.

The analyzer produces this diagnostic when a function invocation is found, but the name being referenced isn't the name of a function, or when the expression computing the function doesn't compute a function.

The following code produces this diagnostic because x isn't a function:

The following code produces this diagnostic because f() doesn't return a function:

If you need to invoke a function, then replace the code before the argument list with the name of a function or with an expression that computes a function:

Can't reference label '{0}' declared in an outer method.

The analyzer produces this diagnostic when a break or continue statement references a label that is declared in a method or function containing the function in which the break or continue statement appears. The break and continue statements can't be used to transfer control outside the function that contains them.

The following code produces this diagnostic because the label loop is declared outside the local function g:

Try rewriting the code so that it isn't necessary to transfer control outside the local function, possibly by inlining the local function:

If that isn't possible, then try rewriting the local function so that a value returned by the function can be used to determine whether control is transferred:

Can't reference an undefined label '{0}'.

The analyzer produces this diagnostic when it finds a reference to a label that isn't defined in the scope of the break or continue statement that is referencing it.

The following code produces this diagnostic because the label loop isn't defined anywhere:

If the label should be on the innermost enclosing do, for, switch, or while statement, then remove the label:

If the label should be on some other statement, then add the label:

Can't have a late final field in a class with a generative const constructor.

The analyzer produces this diagnostic when a class that has at least one const constructor also has a field marked both late and final.

The following code produces this diagnostic because the class A has a const constructor and the final field f is marked as late:

If the field doesn't need to be marked late, then remove the late modifier from the field:

If the field must be marked late, then remove the const modifier from the constructors:

The late final local variable is already assigned.

The analyzer produces this diagnostic when the analyzer can prove that a local variable marked as both late and final was already assigned a value at the point where another assignment occurs.

Because final variables can only be assigned once, subsequent assignments are guaranteed to fail, so they're flagged.

The following code produces this diagnostic because the final variable v is assigned a value in two places:

If you need to be able to reassign the variable, then remove the final keyword:

If you don't need to reassign the variable, then remove all except the first of the assignments:

FFI leaf call can't return a 'Handle'.

The analyzer produces this diagnostic when the value of the isLeaf argument in an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction is true and the function that would be returned would have a return type of Handle.

The analyzer also produces this diagnostic when the value of the isLeaf argument in an Native annotation is true and the type argument on the annotation is a function type whose return type is Handle.

In all of these cases, leaf calls are only supported for the types bool, int, float, double, and, as a return type void.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the function p returns a Handle, but the isLeaf argument is true:

If the function returns a handle, then remove the isLeaf argument:

If the function returns one of the supported types, then correct the type information:

FFI leaf call can't take arguments of type 'Handle'.

The analyzer produces this diagnostic when the value of the isLeaf argument in an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction is true and the function that would be returned would have a parameter of type Handle.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the function p has a parameter of type Handle, but the isLeaf argument is true:

If the function has at least one parameter of type Handle, then remove the isLeaf argument:

If none of the function's parameters are Handles, then correct the type information:

The element type '{0}' can't be assigned to the list type '{1}'.

The analyzer produces this diagnostic when the type of an element in a list literal isn't assignable to the element type of the list.

The following code produces this diagnostic because 2.5 is a double, and the list can hold only integers:

If you intended to add a different object to the list, then replace the element with an expression that computes the intended object:

If the object shouldn't be in the list, then remove the element:

If the object being computed is correct, then widen the element type of the list to allow all of the different types of objects it needs to contain:

The type of the first positional parameter of the 'main' function must be a supertype of 'List'.

The analyzer produces this diagnostic when the first positional parameter of a function named main isn't a supertype of List<String>.

The following code produces this diagnostic because List<int> isn't a supertype of List<String>:

If the function is an entry point, then change the type of the first positional parameter to be a supertype of List<String>:

If the function isn't an entry point, then change the name of the function:

The function 'main' can't have any required named parameters.

The analyzer produces this diagnostic when a function named main has one or more required named parameters.

The following code produces this diagnostic because the function named main has a required named parameter (x):

If the function is an entry point, then remove the required keyword:

If the function isn't an entry point, then change the name of the function:

The function 'main' can't have more than two required positional parameters.

The analyzer produces this diagnostic when a function named main has more than two required positional parameters.

The following code produces this diagnostic because the function main has three required positional parameters:

If the function is an entry point and the extra parameters aren't used, then remove them:

If the function is an entry point, but the extra parameters used are for when the function isn't being used as an entry point, then make the extra parameters optional:

If the function isn't an entry point, then change the name of the function:

The declaration named 'main' must be a function.

The analyzer produces this diagnostic when a library contains a declaration of the name main that isn't the declaration of a top-level function.

The following code produces this diagnostic because the name main is being used to declare a top-level variable:

Use a different name for the declaration:

Map entries can only be used in a map literal.

The analyzer produces this diagnostic when a map entry (a key/value pair) is found in a set literal.

The following code produces this diagnostic because the literal has a map entry even though it's a set literal:

If you intended for the collection to be a map, then change the code so that it is a map. In the previous example, you could do this by adding another type argument:

In other cases, you might need to change the explicit type from Set to Map.

If you intended for the collection to be a set, then remove the map entry, possibly by replacing the colon with a comma if both values should be included in the set:

The element type '{0}' can't be assigned to the map key type '{1}'.

The analyzer produces this diagnostic when a key of a key-value pair in a map literal has a type that isn't assignable to the key type of the map.

The following code produces this diagnostic because 2 is an int, but the keys of the map are required to be Strings:

If the type of the map is correct, then change the key to have the correct type:

If the type of the key is correct, then change the key type of the map:

The element type '{0}' can't be assigned to the map value type '{1}'.

The analyzer produces this diagnostic when a value of a key-value pair in a map literal has a type that isn't assignable to the value type of the map.

The following code produces this diagnostic because 2 is an int, but/ the values of the map are required to be Strings:

If the type of the map is correct, then change the value to have the correct type:

If the type of the value is correct, then change the value type of the map:

The annotation doesn't match the declared type of the field.

The analyzer produces this diagnostic when the annotation on a field in a subclass of Struct or Union doesn't match the Dart type of the field.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the annotation Double doesn't match the Dart type int:

If the type of the field is correct, then change the annotation to match:

If the annotation is correct, then change the type of the field to match:

Fields of type '{0}' in a subclass of '{1}' must have an annotation indicating the native type.

The analyzer produces this diagnostic when a field in a subclass of Struct or Union whose type requires an annotation doesn't have one. The Dart types int, double, and Array are used to represent multiple C types, and the annotation specifies which of the compatible C types the field represents.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field x doesn't have an annotation indicating the underlying width of the integer value:

Add an appropriate annotation to the field:

Required library '{0}' is missing.

The analyzer produces this diagnostic when either the Dart or Flutter SDK isn't installed correctly, and, as a result, one of the dart: libraries can't be found.

Reinstall the Dart or Flutter SDK.

The parameter '{0}' can't have a value of 'null' because of its type, but the implicit default value is 'null'.

With null safety, use the 'required' keyword, not the '@required' annotation.

The analyzer produces this diagnostic when an optional parameter, whether positional or named, has a potentially non-nullable type and doesn't specify a default value. Optional parameters that have no explicit default value have an implicit default value of null. If the type of the parameter doesn't allow the parameter to have a value of null, then the implicit default value isn't valid.

The following code produces this diagnostic because x can't be null, and no non-null default value is specified:

As does this:

If you want to use null to indicate that no value was provided, then you need to make the type nullable:

If the parameter can't be null, then either provide a default value:

or make the parameter a required parameter:

Missing a dependency on imported package '{0}'.

The analyzer produces this diagnostic when there's a package that has been imported in the source but is not listed as a dependency of the importing package.

The following code produces this diagnostic because the package path is not listed as a dependency, while there is an import statement with package path in the source code of package example:

Add the missing package path to the dependencies field:

Missing case clause for '{0}'.

The analyzer produces this diagnostic when a switch statement for an enum doesn't include an option for one of the values in the enum.

Note that null is always a possible value for an enum and therefore also must be handled.

The following code produces this diagnostic because the enum value e2 isn't handled:

If there's special handling for the missing values, then add a case clause for each of the missing values:

If the missing values should be handled the same way, then add a default clause:

The method {0} must have an exceptional return value (the second argument) when the return type of the function is neither 'void', 'Handle', nor 'Pointer'.

The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction or NativeCallable.isolateLocal doesn't have a second argument (the exceptional return value) when the type to be returned from the invocation is neither void, Handle, nor Pointer.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type returned by f is expected to be an 8-bit integer but the call to fromFunction doesn't include an exceptional return argument:

Add an exceptional return type:

Fields in struct classes must have an explicitly declared type of 'int', 'double' or 'Pointer'.

The analyzer produces this diagnostic when a field in a subclass of Struct or Union doesn't have a type annotation. Every field must have an explicit type, and the type must either be int, double, Pointer, or a subclass of either Struct or Union.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field str doesn't have a type annotation:

Explicitly specify the type of the field:

The 'name' field is required but missing.

The analyzer produces this diagnostic when there's no top-level name key. The name key provides the name of the package, which is required.

The following code produces this diagnostic because the package doesn't have a name:

Add the top-level key name with a value that's the name of the package:

The getter name is not specified explicitly, and the pattern is not a variable.

The analyzer produces this diagnostic when, within an object pattern, the specification of a property and the pattern used to match the property's value doesn't have either:

• a getter name before the colon
• a variable pattern from which the getter name can be inferred

The following code produces this diagnostic because there is no getter name before the colon and no variable pattern after the colon in the object pattern (C(:0)):

If you need to use the actual value of the property within the pattern's scope, then add a variable pattern where the name of the variable is the same as the name of the property being matched:

If you don't need to use the actual value of the property within the pattern's scope, then add the name of the property being matched before the colon:

Missing concrete implementation of '{0}'.

Missing concrete implementations of '{0}' and '{1}'.

Missing concrete implementations of '{0}', '{1}', and {2} more.

The analyzer produces this diagnostic when an instance member that has the @mustBeOverridden annotation isn't overridden in a subclass.

The following code produces this diagnostic because the class B doesn't have an override of the inherited method A.m when A.m is annotated with @mustBeOverridden:

If the annotation is appropriate for the member, then override the member in the subclass:

If the annotation isn't appropriate for the member, then remove the annotation:

The named parameter '{0}' is required, but there's no corresponding argument.

The analyzer produces this diagnostic when an invocation of a function is missing a required named parameter.

The following code produces this diagnostic because the invocation of f doesn't include a value for the required named parameter end:

Add a named argument corresponding to the missing required parameter:

The parameter '{0}' is required.

The parameter '{0}' is required. {1}.

The analyzer produces this diagnostic when a method or function with a named parameter that is annotated as being required is invoked without providing a value for the parameter.

The following code produces this diagnostic because the named parameter x is required:

Provide the required value:

This function has a return type of '{0}', but doesn't end with a return statement.

Any function or method that doesn't end with either an explicit return or a throw implicitly returns null. This is rarely the desired behavior. The analyzer produces this diagnostic when it finds an implicit return.

The following code produces this diagnostic because f doesn't end with a return:

Add a return statement that makes the return value explicit, even if null is the appropriate value.

Fields of type 'Array' must have exactly one 'Array' annotation.

The analyzer produces this diagnostic when a field in a subclass of either Struct or Union has a type of Array but doesn't have a single Array annotation indicating the dimensions of the array.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field a0 doesn't have an Array annotation:

Ensure that there's exactly one Array annotation on the field:

Variable pattern '{0}' is missing in this branch of the logical-or pattern.

The analyzer produces this diagnostic when one branch of a logical-or pattern doesn't declare a variable that is declared on the other branch of the same pattern.

The following code produces this diagnostic because the right-hand side of the logical-or pattern doesn't declare the variable a:

If the variable needs to be referenced in the controlled statements, then add a declaration of the variable to every branch of the logical-or pattern:

If the variable doesn't need to be referenced in the controlled statements, then remove the declaration of the variable from every branch of the logical-or pattern:

If the variable needs to be referenced if one branch of the pattern matches but not when the other matches, then break the pattern into two pieces:

The super-invoked member '{0}' has the type '{1}', and the concrete member in the class has the type '{2}'.

The analyzer produces this diagnostic when a mixin that invokes a method using super is used in a class where the concrete implementation of that method has a different signature than the signature defined for that method by the mixin's on type. The reason this is an error is because the invocation in the mixin might invoke the method in a way that's incompatible with the method that will actually be executed.

The following code produces this diagnostic because the class C uses the mixin M, the mixin M invokes foo using super, and the abstract version of foo declared in I (the mixin's on type) doesn't have the same signature as the concrete version of foo declared in A:

If the class doesn't need to use the mixin, then remove it from the with clause:

If the class needs to use the mixin, then ensure that there's a concrete implementation of the method that conforms to the signature expected by the mixin:

'{0}' can't be mixed onto '{1}' because '{1}' doesn't implement '{2}'.

The analyzer produces this diagnostic when a mixin that has a superclass constraint is used in a mixin application with a superclass that doesn't implement the required constraint.

The following code produces this diagnostic because the mixin M requires that the class to which it's applied be a subclass of A, but Object isn't a subclass of A:

If you need to use the mixin, then change the superclass to be either the same as or a subclass of the superclass constraint:

The class doesn't have a concrete implementation of the super-invoked member '{0}'.

The class doesn't have a concrete implementation of the super-invoked setter '{0}'.

The analyzer produces this diagnostic when a mixin application contains an invocation of a member from its superclass, and there's no concrete member of that name in the mixin application's superclass.

The following code produces this diagnostic because the mixin M contains the invocation super.m(), and the class A, which is the superclass of the mixin application A+M, doesn't define a concrete implementation of m:

If you intended to apply the mixin M to a different class, one that has a concrete implementation of m, then change the superclass of B to that class:

If you need to make B a subclass of A, then add a concrete implementation of m in A:

The class '{0}' can't be declared a mixin because it extends a class other than 'Object'.

The analyzer produces this diagnostic when a class that is marked with the mixin modifier extends a class other than Object. A mixin class can't have a superclass other than Object.

The following code produces this diagnostic because the class B, which has the modifier mixin, extends A:

If you want the class to be used as a mixin, then change the superclass to Object, either explicitly or by removing the extends clause:

If the class needs to have a superclass other than Object, then remove the mixin modifier:

If you need both a mixin and a subclass of a class other than Object, then move the members of the subclass to a new mixin, remove the mixin modifier from the subclass, and apply the new mixin to the subclass:

Depending on the members of the subclass this might require adding an on clause to the mixin.

The class '{0}' can't be used as a mixin because it declares a constructor.

The analyzer produces this diagnostic when a class is used as a mixin and the mixed-in class defines a constructor.

The following code produces this diagnostic because the class A, which defines a constructor, is being used as a mixin:

If it's possible to convert the class to a mixin, then do so:

If the class can't be a mixin and it's possible to remove the constructor, then do so:

If the class can't be a mixin and you can't remove the constructor, then try extending or implementing the class rather than mixing it in:

The class '{0}' can't be used as a mixin because it extends a class other than 'Object'.

The analyzer produces this diagnostic when a class that extends a class other than Object is used as a mixin.

The following code produces this diagnostic because the class B, which extends A, is being used as a mixin by C:

If the class being used as a mixin can be changed to extend Object, then change it:

If the class being used as a mixin can't be changed and the class that's using it extends Object, then extend the class being used as a mixin:

If the class doesn't extend Object or if you want to be able to mix in the behavior from B in other places, then create a real mixin:

Mixins can't be instantiated.

The analyzer produces this diagnostic when a mixin is instantiated.

The following code produces this diagnostic because the mixin M is being instantiated:

If you intend to use an instance of a class, then use the name of that class in place of the name of the mixin.

Classes can only mix in mixins and classes.

The analyzer produces this diagnostic when a name in a with clause is defined to be something other than a mixin or a class.

The following code produces this diagnostic because F is defined to be a function type:

Remove the invalid name from the list, possibly replacing it with the name of the intended mixin or class:

The class '{0}' shouldn't be used as a mixin constraint because it is sealed, and any class mixing in this mixin must have '{0}' as a superclass.

The analyzer produces this diagnostic when the superclass constraint of a mixin is a class from a different package that was marked as sealed. Classes that are sealed can't be extended, implemented, mixed in, or used as a superclass constraint.

If the package p defines a sealed class:

Then, the following code, when in a package other than p, produces this diagnostic:

If the classes that use the mixin don't need to be subclasses of the sealed class, then consider adding a field and delegating to the wrapped instance of the sealed class.

Deferred classes can't be used as superclass constraints.

The analyzer produces this diagnostic when a superclass constraint of a mixin is imported from a deferred library.

The following code produces this diagnostic because the superclass constraint of math.Random is imported from a deferred library:

If the import doesn't need to be deferred, then remove the deferred keyword:

If the import does need to be deferred, then remove the superclass constraint:

Only classes and mixins can be used as superclass constraints.

The analyzer produces this diagnostic when a type following the on keyword in a mixin declaration is neither a class nor a mixin.

The following code produces this diagnostic because F is neither a class nor a mixin:

If the type was intended to be a class but was mistyped, then replace the name.

Otherwise, remove the type from the on clause.

Using multiple 'hide' or 'show' combinators is never necessary and often produces surprising results.

The analyzer produces this diagnostic when an import or export directive contains more than one combinator.

The following code produces this diagnostic because the second show combinator hides List and int:

The following code produces this diagnostic because the second hide combinator is redundant:

The following codes produce this diagnostic because the hide combinator is redundant:

The following code produces this diagnostic because the show combinator already hides Random and max, so the hide combinator is redundant:

If you prefer to list the names that should be visible, then use a single show combinator:

If you prefer to list the names that should be hidden, then use a single hide combinator:

Constructors can have only one 'this' redirection, at most.

The analyzer produces this diagnostic when a constructor redirects to more than one other constructor in the same class (using this).

The following code produces this diagnostic because the unnamed constructor in C is redirecting to both this.a and this.b:

Remove all but one of the redirections:

A constructor can have at most one 'super' initializer.

The analyzer produces this diagnostic when the initializer list of a constructor contains more than one invocation of a constructor from the superclass. The initializer list is required to have exactly one such call, which can either be explicit or implicit.

The following code produces this diagnostic because the initializer list for B's constructor invokes both the constructor one and the constructor two from the superclass A:

If one of the super constructors will initialize the instance fully, then remove the other:

If the initialization achieved by one of the super constructors can be performed in the body of the constructor, then remove its super invocation and perform the initialization in the body:

If the initialization can only be performed in a constructor in the superclass, then either add a new constructor or modify one of the existing constructors so there's a constructor that allows all the required initialization to occur in a single call:

The type '{0}' given to '{1}' must be a valid 'dart:ffi' native function type.

The analyzer produces this diagnostic when an invocation of either Pointer.fromFunction, DynamicLibrary.lookupFunction, or a NativeCallable constructor, has a type argument(whether explicit or inferred) that isn't a native function type.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type T can be any subclass of Function but the type argument for fromFunction is required to be a native function type:

Use a native function type as the type argument to the invocation:

The type '{0}' must be a subtype of '{1}' for '{2}'.

The analyzer produces this diagnostic in two cases:

• In an invocation of Pointer.fromFunction, or a NativeCallable constructor where the type argument (whether explicit or inferred) isn't a supertype of the type of the function passed as the first argument to the method.
• In an invocation of DynamicLibrary.lookupFunction where the first type argument isn't a supertype of the second type argument.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type of the function f (String Function(int)) isn't a subtype of the type argument T (Int8 Function(Int8)):

If the function is correct, then change the type argument to match:

If the type argument is correct, then change the function to match:

This class (or a class that this class inherits from) is marked as '@immutable', but one or more of its instance fields aren't final: {0}

The analyzer produces this diagnostic when an immutable class defines one or more instance fields that aren't final. A class is immutable if it's marked as being immutable using the annotation immutable or if it's a subclass of an immutable class.

The following code produces this diagnostic because the field x isn't final:

If instances of the class should be immutable, then add the keyword final to all non-final field declarations:

If the instances of the class should be mutable, then remove the annotation, or choose a different superclass if the annotation is inherited:

This method overrides a method annotated as '@mustCallSuper' in '{0}', but doesn't invoke the overridden method.

The analyzer produces this diagnostic when a method that overrides a method that is annotated as mustCallSuper doesn't invoke the overridden method as required.

The following code produces this diagnostic because the method m in B doesn't invoke the overridden method m in A:

Add an invocation of the overridden method in the overriding method:

The return type of the function passed to 'NativeCallable.listener' must be 'void' rather than '{0}'.

The analyzer produces this diagnostic when you pass a function that doesn't return void to the NativeCallable.listener constructor.

NativeCallable.listener creates a native callable that can be invoked from any thread. The native code that invokes the callable sends a message back to the isolate that created the callable, and doesn't wait for a response. So it isn't possible to return a result from the callable.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the function f returns int rather than void.

Change the return type of the function to void.

The value of the 'name' field is required to be a string.

The analyzer produces this diagnostic when the top-level name key has a value that isn't a string.

The following code produces this diagnostic because the value following the name key is a list:

Replace the value with a string:

'{0}' is an unsupported type for native fields. Native fields only support pointers, arrays or numeric and compound types.

The analyzer produces this diagnostic when an @Native-annotated field has a type not supported for native fields.

Native fields support pointers, arrays, numeric types and subtypes of Compound (i.e., structs or unions). Other subtypes of NativeType, such as Handle or NativeFunction are not allowed as native fields.

Native functions should be used with external functions instead of external fields.

Handles are unsupported because there is no way to transparently load and store Dart objects into pointers.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field free uses an unsupported native type, NativeFunction:

If you meant to bind to an existing native function with a NativeFunction field, use @Native methods instead:

To bind to a field storing a function pointer in C, use a pointer type for the Dart field:

The native type of this field could not be inferred and must be specified in the annotation.

The analyzer produces this diagnostic when an @Native-annotated field requires a type hint on the annotation to infer the native type.

Dart types like int and double have multiple possible native representations. Since the native type needs to be known at compile time to generate the correct load and stores when accessing the field, an explicit type must be given.

The following code produces this diagnostic because the field f has the type int (for which multiple native representations exist), but no explicit type parameter on the Native annotation:

To fix this diagnostic, find out the correct native representation from the native declaration of the field. Then, add the corresponding type to the annotation. For instance, if f was declared as an uint8_t in C, the Dart field should be declared as:

For more information about FFI, see C interop using dart:ffi.

Native fields must be static.

The analyzer produces this diagnostic when an instance field in a class has been annotated with @Native. Native fields refer to global variables in C, C++ or other native languages, whereas instance fields in Dart are specific to an instance of that class. Hence, native fields must be static.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field f in the class C is @Native, but not static:

Either make the field static:

Or move it out of a class, in which case no explicit static modifier is required:

If you meant to annotate an instance field that should be part of a struct, omit the @Native annotation:

The native type of this function couldn't be inferred so it must be specified in the annotation.

The analyzer produces this diagnostic when a @Native-annotated function requires a type hint on the annotation to infer the native function type.

Dart types like int and double have multiple possible native representations. Since the native type needs to be known at compile time to generate correct bindings and call instructions for the function, an explicit type must be given.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the function f() has the return type int, but doesn't have an explicit type parameter on the Native annotation:

Add the corresponding type to the annotation. For instance, if f() was declared to return an int32_t in C, the Dart function should be declared as:

The variable dimension of a variable-length array must be non-negative.

The analyzer produces this diagnostic in two cases.

The first is when the variable dimension given in an Array.variableWithVariableDimension annotation is negative. The variable dimension is the first argument in the annotation.

The second is when the variable dimension given in an Array.variableMulti annotation is negative. The variable dimension is specified in the variableDimension argument of the annotation.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because a variable dimension of -1 was provided in the Array.variableWithVariableDimension annotation:

The following code produces this diagnostic because a variable dimension of -1 was provided in the Array.variableMulti annotation:

Change the variable dimension with zero (0) or a positive number:

Change the variable dimension with zero (0) or a positive number:

The class '{0}' doesn't have an unnamed constructor.

The analyzer produces this diagnostic when an unnamed constructor is invoked on a class that defines named constructors but the class doesn't have an unnamed constructor.

The following code produces this diagnostic because A doesn't define an unnamed constructor:

If one of the named constructors does what you need, then use it:

If none of the named constructors does what you need, and you're able to add an unnamed constructor, then add the constructor:

Missing concrete implementation of '{0}'.

Missing concrete implementations of '{0}' and '{1}'.

Missing concrete implementations of '{0}', '{1}', '{2}', '{3}', and {4} more.

Missing concrete implementations of '{0}', '{1}', '{2}', and '{3}'.

Missing concrete implementations of '{0}', '{1}', and '{2}'.

The analyzer produces this diagnostic when a concrete class inherits one or more abstract members, and doesn't provide or inherit an implementation for at least one of those abstract members.

The following code produces this diagnostic because the class B doesn't have a concrete implementation of m:

If the subclass can provide a concrete implementation for some or all of the abstract inherited members, then add the concrete implementations:

If there is a mixin that provides an implementation of the inherited methods, then apply the mixin to the subclass:

If the subclass can't provide a concrete implementation for all of the abstract inherited members, then mark the subclass as being abstract:

Conditions must have a static type of 'bool'.

The analyzer produces this diagnostic when a condition, such as an if or while loop, doesn't have the static type bool.

The following code produces this diagnostic because x has the static type int:

Change the condition so that it produces a Boolean value:

The expression in an assert must be of type 'bool'.

The analyzer produces this diagnostic when the first expression in an assert has a type other than bool.

The following code produces this diagnostic because the type of p is int, but a bool is required:

Change the expression so that it has the type bool:

A negation operand must have a static type of 'bool'.

The analyzer produces this diagnostic when the operand of the unary negation operator (!) doesn't have the type bool.

The following code produces this diagnostic because x is an int when it must be a bool:

Replace the operand with an expression that has the type bool:

The operands of the operator '{0}' must be assignable to 'bool'.

The analyzer produces this diagnostic when one of the operands of either the && or || operator doesn't have the type bool.

The following code produces this diagnostic because a isn't a Boolean value:

Change the operand to a Boolean value:

Annotation creation can only call a const constructor.

The analyzer produces this diagnostic when an annotation is the invocation of an existing constructor even though the invoked constructor isn't a const constructor.

The following code produces this diagnostic because the constructor for C isn't a const constructor:

If it's valid for the class to have a const constructor, then create a const constructor that can be used for the annotation:

If it isn't valid for the class to have a const constructor, then either remove the annotation or use a different class for the annotation.

Case expressions must be constant.

The analyzer produces this diagnostic when the expression in a case clause isn't a constant expression.

The following code produces this diagnostic because j isn't a constant:

Either make the expression a constant expression, or rewrite the switch statement as a sequence of if statements:

Constant values from a deferred library can't be used as a case expression.

The analyzer produces this diagnostic when the expression in a case clause references a constant from a library that is imported using a deferred import. In order for switch statements to be compiled efficiently, the constants referenced in case clauses need to be available at compile time, and constants from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Given a file a.dart that defines the constant zero:

The following code produces this diagnostic because the library a.dart is imported using a deferred import, and the constant a.zero, declared in the imported library, is used in a case clause:

If you need to reference the constant from the imported library, then remove the deferred keyword:

If you need to reference the constant from the imported library and also need the imported library to be deferred, then rewrite the switch statement as a sequence of if statements:

If you don't need to reference the constant, then replace the case expression:

The default value of an optional parameter must be constant.

The analyzer produces this diagnostic when an optional parameter, either named or positional, has a default value that isn't a compile-time constant.

The following code produces this diagnostic:

If the default value can be converted to be a constant, then convert it:

If the default value needs to change over time, then apply the default value inside the function:

Constant values from a deferred library can't be used as a default parameter value.

The analyzer produces this diagnostic when the default value of an optional parameter uses a constant from a library imported using a deferred import. Default values need to be available at compile time, and constants from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Given a file a.dart that defines the constant zero:

The following code produces this diagnostic because zero is declared in a library imported using a deferred import:

If you need to reference the constant from the imported library, then remove the deferred keyword:

If you don't need to reference the constant, then replace the default value:

The values in a const list literal must be constants.

The analyzer produces this diagnostic when an element in a constant list literal isn't a constant value. The list literal can be constant either explicitly (because it's prefixed by the const keyword) or implicitly (because it appears in a constant context).

The following code produces this diagnostic because x isn't a constant, even though it appears in an implicitly constant list literal:

If the list needs to be a constant list, then convert the element to be a constant. In the example above, you might add the const keyword to the declaration of x:

If the expression can't be made a constant, then the list can't be a constant either, so you must change the code so that the list isn't a constant. In the example above this means removing the const keyword before the list literal:

The elements in a const map literal must be constant.

The analyzer produces this diagnostic when an if element or a spread element in a constant map isn't a constant element.

The following code produces this diagnostic because it's attempting to spread a non-constant map:

Similarly, the following code produces this diagnostic because the condition in the if element isn't a constant expression:

If the map needs to be a constant map, then make the elements constants. In the spread example, you might do that by making the collection being spread a constant:

If the map doesn't need to be a constant map, then remove the const keyword:

The keys in a const map literal must be constant.

The analyzer produces this diagnostic when a key in a constant map literal isn't a constant value.

The following code produces this diagnostic because a isn't a constant:

If the map needs to be a constant map, then make the key a constant:

If the map doesn't need to be a constant map, then remove the const keyword:

Key expressions in map patterns must be constants.

The analyzer produces this diagnostic when a key in a map pattern isn't a constant expression.

The following code produces this diagnostic because the key A() isn't a constant:

Use a constant for the key:

The values in a const map literal must be constant.

The analyzer produces this diagnostic when a value in a constant map literal isn't a constant value.

The following code produces this diagnostic because a isn't a constant:

If the map needs to be a constant map, then make the key a constant:

If the map doesn't need to be a constant map, then remove the const keyword:

The relational pattern expression must be a constant.

The analyzer produces this diagnostic when the value in a relational pattern expression isn't a constant expression.

The following code produces this diagnostic because the operand of the > operator, a, isn't a constant:

Replace the value with a constant expression:

The values in a const set literal must be constants.

The analyzer produces this diagnostic when a constant set literal contains an element that isn't a compile-time constant.

The following code produces this diagnostic because i isn't a constant:

If the element can be changed to be a constant, then change it:

If the element can't be a constant, then remove the keyword const:

The type arguments to '{0}' must be known at compile time, so they can't be type parameters.

The analyzer produces this diagnostic when the type arguments to a method are required to be known at compile time, but a type parameter, whose value can't be known at compile time, is used as a type argument.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type argument to Pointer.asFunction must be known at compile time, but the type parameter R, which isn't known at compile time, is being used as the type argument:

Remove any uses of type parameters:

Argument '{0}' must be a constant.

The analyzer produces this diagnostic when a parameter is annotated with the mustBeConst annotation and the corresponding argument is not a constant expression.

The following code produces this diagnostic on the invocation of the function f because the value of the argument passed to the function g isn't a constant:

If a suitable constant is available to use, then replace the argument with a constant:

This instance creation must be 'const', because the {0} constructor is marked as '@literal'.

The analyzer produces this diagnostic when a constructor that has the literal annotation is invoked without using the const keyword, but all of the arguments to the constructor are constants. The annotation indicates that the constructor should be used to create a constant value whenever possible.

The following code produces this diagnostic:

Add the keyword const before the constructor invocation:

Generative enum constructors must be 'const'.

The analyzer produces this diagnostic when an enum declaration contains a generative constructor that isn't marked as const.

The following code produces this diagnostic because the constructor in E isn't marked as being const:

Add the const keyword before the constructor:

An extension type parameter can't be used in a non-covariant position of its representation type.

The analyzer produces this diagnostic when a type parameter of an extension type is used in a non-covariant position in the representation type of that extension type.

The following code produces this diagnostic because the type parameter T is used as a parameter type in the function type void Function(T), and parameters are not covariant:

Remove the use of the type parameter:

The type '{0}' is not exhaustively matched by the switch cases since it doesn't match '{1}'.

The analyzer produces this diagnostic when a switch expression is missing a case for one or more of the possible values that could flow through it.

The following code produces this diagnostic because the switch expression doesn't have a case for the value E.three:

If the missing values are distinctly meaningful to the switch expression, then add a case for each of the values missing a match:

If the missing values don't need to be matched, then add a wildcard pattern that returns a simple default:

Be aware that a wildcard pattern will handle any values added to the type in the future. You will lose the ability to have the compiler warn you if the switch needs to be updated to account for newly added types.

The type '{0}' is not exhaustively matched by the switch cases since it doesn't match '{1}'.

The analyzer produces this diagnostic when a switch statement switching over an exhaustive type is missing a case for one or more of the possible values that could flow through it.

The following code produces this diagnostic because the switch statement doesn't have a case for the value E.three, and E is an exhaustive type:

Add a case for each of the constants that aren't currently being matched:

If the missing values don't need to be matched, then add a default clause or a wildcard pattern:

But be aware that adding a default clause or wildcard pattern will cause any future values of the exhaustive type to also be handled, so you will have lost the ability for the compiler to warn you if the switch needs to be updated.

Enums can only declare final fields.

The analyzer produces this diagnostic when an instance field in an enum isn't marked as final.

The following code produces this diagnostic because the field f isn't a final field:

If the field must be defined for the enum, then mark the field as being final:

If the field can be removed, then remove it:

The generative constructor '{0}' is expected, but a factory was found.

The analyzer produces this diagnostic when the initializer list of a constructor invokes a constructor from the superclass, and the invoked constructor is a factory constructor. Only a generative constructor can be invoked in the initializer list.

The following code produces this diagnostic because the invocation of the constructor super.one() is invoking a factory constructor:

Change the super invocation to invoke a generative constructor:

If the generative constructor is the unnamed constructor, and if there are no arguments being passed to it, then you can remove the super invocation.

The unnamed constructor of superclass '{0}' (called by the default constructor of '{1}') must be a generative constructor, but factory found.

The analyzer produces this diagnostic when a class has an implicit generative constructor and the superclass has an explicit unnamed factory constructor. The implicit constructor in the subclass implicitly invokes the unnamed constructor in the superclass, but generative constructors can only invoke another generative constructor, not a factory constructor.

The following code produces this diagnostic because the implicit constructor in B invokes the unnamed constructor in A, but the constructor in A is a factory constructor, when a generative constructor is required:

If the unnamed constructor in the superclass can be a generative constructor, then change it to be a generative constructor:

If the unnamed constructor can't be a generative constructor and there are other generative constructors in the superclass, then explicitly invoke one of them:

If there are no generative constructors that can be used and none can be added, then implement the superclass rather than extending it:

Can't invoke 'asFunction' because the function signature '{0}' for the pointer isn't a valid C function signature.

The analyzer produces this diagnostic when the method asFunction is invoked on a pointer to a native function, but the signature of the native function isn't a valid C function signature.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because function signature associated with the pointer p (FNative) isn't a valid C function signature:

Make the NativeFunction signature a valid C signature:

Array dimensions must be positive numbers.

The analyzer produces this diagnostic when a dimension given in an Array annotation is less than or equal to zero (0).

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because an array dimension of -8 was provided:

Change the dimension to be a positive integer:

If this is a variable length inline array, change the annotation to Array.variable():

The type '{1}' isn't a valid type argument for '{0}'. The type argument must be a native integer, 'Float', 'Double', 'Pointer', or subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The analyzer produces this diagnostic when the type argument for the class Array isn't one of the valid types: either a native integer, Float, Double, Pointer, or subtype of Struct, Union, or AbiSpecificInteger.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type argument to Array is Void, and Void isn't one of the valid types:

Change the type argument to one of the valid types:

Factory bodies can't use 'async', 'async*', or 'sync*'.

The analyzer produces this diagnostic when the body of a factory constructor is marked with async, async*, or sync*. All constructors, including factory constructors, are required to return an instance of the class in which they're declared, not a Future, Stream, or Iterator.

The following code produces this diagnostic because the body of the factory constructor is marked with async:

If the member must be declared as a factory constructor, then remove the keyword appearing before the body:

If the member must return something other than an instance of the enclosing class, then make the member a static method:

The name '{0}' isn't a type, so it can't be used as a type argument.

The analyzer produces this diagnostic when an identifier that isn't a type is used as a type argument.

The following code produces this diagnostic because x is a variable, not a type:

Change the type argument to be a type:

The name '{0}' isn't a type and can't be used in an on-catch clause.

The analyzer produces this diagnostic when the identifier following the on in a catch clause is defined to be something other than a type.

The following code produces this diagnostic because f is a function, not a type:

Change the name to the type of object that should be caught:

The return type of the operator []= must be 'void'.

The analyzer produces this diagnostic when a declaration of the operator []= has a return type other than void.

The following code produces this diagnostic because the declaration of the operator []= has a return type of int:

Change the return type to void:

The return type of the setter must be 'void' or absent.

The analyzer produces this diagnostic when a setter is defined with a return type other than void.

The following code produces this diagnostic because the setter p has a return type of int:

Change the return type to void or omit the return type:

The non-nullable local variable '{0}' must be assigned before it can be used.

The analyzer produces this diagnostic when a local variable is referenced and has all these characteristics:

• Has a type that's potentially non-nullable.
• Doesn't have an initializer.
• Isn't marked as late.
• The analyzer can't prove that the local variable will be assigned before the reference based on the specification of definite assignment.

The following code produces this diagnostic because x can't have a value of null, but is referenced before a value was assigned to it:

The following code produces this diagnostic because the assignment to x might not be executed, so it might have a value of null:

The following code produces this diagnostic because the analyzer can't prove, based on definite assignment analysis, that x won't be referenced without having a value assigned to it:

If null is a valid value, then make the variable nullable:

If null isn't a valid value, and there's a reasonable default value, then add an initializer:

Otherwise, ensure that a value was assigned on every possible code path before the value is accessed:

You can also mark the variable as late, which removes the diagnostic, but if the variable isn't assigned a value before it's accessed, then it results in an exception being thrown at runtime. This approach should only be used if you're sure that the variable will always be assigned, even though the analyzer can't prove it based on definite assignment analysis.

{0} isn't a type.

The analyzer produces this diagnostic when a name is used as a type but declared to be something other than a type.

The following code produces this diagnostic because f is a function:

Replace the name with the name of a type.

'{0}' isn't a binary operator.

The analyzer produces this diagnostic when an operator that can only be used as a unary operator is used as a binary operator.

The following code produces this diagnostic because the operator ~ can only be used as a unary operator:

Replace the operator with the correct binary operator:

1 positional argument expected by '{0}', but 0 found.

1 positional argument expected, but 0 found.

{0} positional arguments expected by '{2}', but {1} found.

{0} positional arguments expected, but {1} found.

The analyzer produces this diagnostic when a method or function invocation has fewer positional arguments than the number of required positional parameters.

The following code produces this diagnostic because f declares two required parameters, but only one argument is provided:

Add arguments corresponding to the remaining parameters:

Non-nullable instance field '{0}' must be initialized.

The analyzer produces this diagnostic when a field is declared and has all these characteristics:

• Has a type that's potentially non-nullable
• Doesn't have an initializer
• Isn't marked as late

The following code produces this diagnostic because x is implicitly initialized to null when it isn't allowed to be null:

Similarly, the following code produces this diagnostic because x is implicitly initialized to null, when it isn't allowed to be null, by one of the constructors, even though it's initialized by other constructors:

If there's a reasonable default value for the field that's the same for all instances, then add an initializer expression:

If the value of the field should be provided when an instance is created, then add a constructor that sets the value of the field or update an existing constructor:

You can also mark the field as late, which removes the diagnostic, but if the field isn't assigned a value before it's accessed, then it results in an exception being thrown at runtime. This approach should only be used if you're sure that the field will always be assigned before it's referenced.

The non-nullable variable '{0}' must be initialized.

The analyzer produces this diagnostic when a static field or top-level variable has a type that's non-nullable and doesn't have an initializer. Fields and variables that don't have an initializer are normally initialized to null, but the type of the field or variable doesn't allow it to be set to null, so an explicit initializer must be provided.

The following code produces this diagnostic because the field f can't be initialized to null:

Similarly, the following code produces this diagnostic because the top-level variable v can't be initialized to null:

If the field or variable can't be initialized to null, then add an initializer that sets it to a non-null value:

If the field or variable should be initialized to null, then change the type to be nullable:

If the field or variable can't be initialized in the declaration but will always be initialized before it's referenced, then mark it as being late:

Spread elements in list or set literals must implement 'Iterable'.

The analyzer produces this diagnostic when the static type of the expression of a spread element that appears in either a list literal or a set literal doesn't implement the type Iterable.

The following code produces this diagnostic:

The most common fix is to replace the expression with one that produces an iterable object:

Spread elements in map literals must implement 'Map'.

The analyzer produces this diagnostic when the static type of the expression of a spread element that appears in a map literal doesn't implement the type Map.

The following code produces this diagnostic because l isn't a Map:

The most common fix is to replace the expression with one that produces a map:

Annotation creation must have arguments.

The analyzer produces this diagnostic when an annotation consists of a single identifier, but that identifier is the name of a class rather than a variable. To create an instance of the class, the identifier must be followed by an argument list.

The following code produces this diagnostic because C is a class, and a class can't be used as an annotation without invoking a const constructor from the class:

Add the missing argument list:

Can't infer missing types in '{0}' from overridden methods: {1}.

The analyzer produces this diagnostic when there is a method declaration for which one or more types needs to be inferred, and those types can't be inferred because none of the overridden methods has a function type that is a supertype of all the other overridden methods, as specified by override inference.

The following code produces this diagnostic because the method m declared in the class C is missing both the return type and the type of the parameter a, and neither of the missing types can be inferred for it:

In this example, override inference can't be performed because the overridden methods are incompatible in these ways:

• Neither parameter type (String and int) is a supertype of the other.
• Neither return type is a subtype of the other.

If possible, add types to the method in the subclass that are consistent with the types from all the overridden methods:

The class '{0}' can't extend '{1}' because '{1}' only has factory constructors (no generative constructors), and '{0}' has at least one generative constructor.

The analyzer produces this diagnostic when a class that has at least one generative constructor (whether explicit or implicit) has a superclass that doesn't have any generative constructors. Every generative constructor, except the one defined in Object, invokes, either explicitly or implicitly, one of the generative constructors from its superclass.

The following code produces this diagnostic because the class B has an implicit generative constructor that can't invoke a generative constructor from A because A doesn't have any generative constructors:

If the superclass should have a generative constructor, then add one:

If the subclass shouldn't have a generative constructor, then remove it by adding a factory constructor:

If the subclass must have a generative constructor but the superclass can't have one, then implement the superclass instead:

A potentially nullable type can't be used in an 'on' clause because it isn't valid to throw a nullable expression.

The analyzer produces this diagnostic when the type following on in a catch clause is a nullable type. It isn't valid to specify a nullable type because it isn't possible to catch null (because it's a runtime error to throw null).

The following code produces this diagnostic because the exception type is specified to allow null when null can't be thrown:

Remove the question mark from the type:

A class can't extend a nullable type.

The analyzer produces this diagnostic when a class declaration uses an extends clause to specify a superclass, and the superclass is followed by a ?.

It isn't valid to specify a nullable superclass because doing so would have no meaning; it wouldn't change either the interface or implementation being inherited by the class containing the extends clause.

Note, however, that it is valid to use a nullable type as a type argument to the superclass, such as class A extends B<C?> {}.

The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in an extends clause:

Remove the question mark from the type:

A class, mixin, or extension type can't implement a nullable type.

The analyzer produces this diagnostic when a class, mixin, or extension type declaration has an implements clause, and an interface is followed by a ?.

It isn't valid to specify a nullable interface because doing so would have no meaning; it wouldn't change the interface being inherited by the class containing the implements clause.

Note, however, that it is valid to use a nullable type as a type argument to the interface, such as class A implements B<C?> {}.

The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in an implements clause:

Remove the question mark from the type:

A mixin can't have a nullable type as a superclass constraint.

The analyzer produces this diagnostic when a mixin declaration uses an on clause to specify a superclass constraint, and the class that's specified is followed by a ?.

It isn't valid to specify a nullable superclass constraint because doing so would have no meaning; it wouldn't change the interface being depended on by the mixin containing the on clause.

Note, however, that it is valid to use a nullable type as a type argument to the superclass constraint, such as mixin A on B<C?> {}.

The following code produces this diagnostic because A? is a nullable type and nullable types can't be used in an on clause:

Remove the question mark from the type:

A class or mixin can't mix in a nullable type.

The analyzer produces this diagnostic when a class or mixin declaration has a with clause, and a mixin is followed by a ?.

It isn't valid to specify a nullable mixin because doing so would have no meaning; it wouldn't change either the interface or implementation being inherited by the class containing the with clause.

Note, however, that it is valid to use a nullable type as a type argument to the mixin, such as class A with B<C?> {}.

The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in a with clause:

Remove the question mark from the type:

'{0}' shouldn't be called with a 'null' argument for the non-nullable type argument '{1}'.

The analyzer produces this diagnostic when null is passed to either the constructor Future.value or the method Completer.complete when the type argument used to create the instance was non-nullable. Even though the type system can't express this restriction, passing in a null results in a runtime exception.

The following code produces this diagnostic because null is being passed to the constructor Future.value even though the type argument is the non-nullable type String:

Pass in a non-null value:

This null-check will always throw an exception because the expression will always evaluate to 'null'.

The analyzer produces this diagnostic when the null check operator (!) is used on an expression whose value can only be null. In such a case the operator always throws an exception, which likely isn't the intended behavior.

The following code produces this diagnostic because the function g will always return null, which means that the null check in f will always throw:

If you intend to always throw an exception, then replace the null check with an explicit throw expression to make the intent more clear:

Using a colon as the separator before a default value is no longer supported.

The analyzer produces this diagnostic when a colon (:) is used as the separator before the default value of an optional named parameter. While this syntax used to be allowed, it was removed in favor of using an equal sign (=).

The following code produces this diagnostic because a colon is being used before the default value of the optional parameter i:

Replace the colon with an equal sign:

The type '{0}' can be included in the superclass constraints only once.

The analyzer produces this diagnostic when the same type is listed in the superclass constraints of a mixin multiple times.

The following code produces this diagnostic because A is included twice in the superclass constraints for M:

If a different type should be included in the superclass constraints, then replace one of the occurrences with the other type:

If no other type was intended, then remove the repeated type name:

Optional parameters aren't allowed when defining an operator.

The analyzer produces this diagnostic when one or more of the parameters in an operator declaration are optional.

The following code produces this diagnostic because the parameter other is an optional parameter:

Make all of the parameters be required parameters:

The field doesn't override an inherited getter or setter.

The getter doesn't override an inherited getter.

The method doesn't override an inherited method.

The setter doesn't override an inherited setter.

The analyzer produces this diagnostic when a class member is annotated with the @override annotation, but the member isn't declared in any of the supertypes of the class.

The following code produces this diagnostic because m isn't declared in any of the supertypes of C:

If the member is intended to override a member with a different name, then update the member to have the same name:

If the member is intended to override a member that was removed from the superclass, then consider removing the member from the subclass.

If the member can't be removed, then remove the annotation.

Structs must have at most one 'Packed' annotation.

The analyzer produces this diagnostic when a subclass of Struct has more than one Packed annotation.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C, which is a subclass of Struct, has two Packed annotations:

Remove all but one of the annotations:

Only packing to 1, 2, 4, 8, and 16 bytes is supported.

The analyzer produces this diagnostic when the argument to the Packed annotation isn't one of the allowed values: 1, 2, 4, 8, or 16.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the argument to the Packed annotation (3) isn't one of the allowed values:

Change the alignment to be one of the allowed values:

Expected this library to be part of '{0}', not '{1}'.

The analyzer produces this diagnostic when a library attempts to include a file as a part of itself when the other file is a part of a different library.

Given a file part.dart containing

The following code, in any file other than library.dart, produces this diagnostic because it attempts to include part.dart as a part of itself when part.dart is a part of a different library:

If the library should be using a different file as a part, then change the URI in the part directive to be the URI of the other file.

If the part file should be a part of this library, then update the URI (or library name) in the part-of directive to be the URI (or name) of the correct library.

The included part '{0}' must have a part-of directive.

The analyzer produces this diagnostic when a part directive is found and the referenced file doesn't have a part-of directive.

Given a file a.dart containing:

The following code produces this diagnostic because a.dart doesn't contain a part-of directive:

If the referenced file is intended to be a part of another library, then add a part-of directive to the file:

If the referenced file is intended to be a library, then replace the part directive with an import directive:

The library is unnamed. A URI is expected, not a library name '{0}', in the part-of directive.

The analyzer produces this diagnostic when a library that doesn't have a library directive (and hence has no name) contains a part directive and the part of directive in the part file uses a name to specify the library that it's a part of.

Given a part file named part_file.dart containing the following code:

The following code produces this diagnostic because the library including the part file doesn't have a name even though the part file uses a name to specify which library it's a part of:

Change the part of directive in the part file to specify its library by URI:

The path '{0}' doesn't exist.

The analyzer produces this diagnostic when a dependency has a path key referencing a directory that doesn't exist.

Assuming that the directory doesNotExist doesn't exist, the following code produces this diagnostic because it's listed as the path of a package:

If the path is correct, then create a directory at that path.

If the path isn't correct, then change the path to match the path to the root of the package.

The path '{0}' isn't a POSIX-style path.

The analyzer produces this diagnostic when a dependency has a path key whose value is a string, but isn't a POSIX-style path.

The following code produces this diagnostic because the path following the path key is a Windows path:

Convert the path to a POSIX path.

The directory '{0}' doesn't contain a pubspec.

The analyzer produces this diagnostic when a dependency has a path key that references a directory that doesn't contain a pubspec.yaml file.

Assuming that the directory local_package doesn't contain a file pubspec.yaml, the following code produces this diagnostic because it's listed as the path of a package:

If the path is intended to be the root of a package, then add a pubspec.yaml file in the directory:

If the path is wrong, then replace it with the correct path.

Only local variables can be assigned in pattern assignments.

The analyzer produces this diagnostic when a pattern assignment assigns a value to anything other than a local variable. Patterns can't assign to fields or top-level variables.

If the code is cleaner when destructuring with a pattern, then rewrite the code to assign the value to a local variable in a pattern declaration, assigning the non-local variable separately:

If the code is cleaner when using a pattern assignment, then rewrite the code to assign the value to a local variable, assigning the non-local variable separately:

If the code is cleaner without using a pattern assignment, then rewrite the code to not use a pattern assignment:

Constant values from a deferred library can't be used in patterns.

The analyzer produces this diagnostic when a pattern contains a value declared in a different library, and that library is imported using a deferred import. Constants are evaluated at compile time, but values from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Given a file a.dart that defines the constant zero:

The following code produces this diagnostic because the constant pattern a.zero is imported using a deferred import:

If you need to reference the constant from the imported library, then remove the deferred keyword:

If you need to reference the constant from the imported library and also need the imported library to be deferred, then rewrite the switch statement as a sequence of if statements:

If you don't need to reference the constant, then replace the case expression:

The matched value of type '{0}' isn't assignable to the required type '{1}'.

The analyzer produces this diagnostic when the type of the value on the right-hand side of a pattern assignment or pattern declaration doesn't match the type required by the pattern being used to match it.

The following code produces this diagnostic because x might not be a String and hence might not match the object pattern:

Change the code so that the type of the expression on the right-hand side matches the type required by the pattern:

Pattern variables can't be assigned inside the guard of the enclosing guarded pattern.

The analyzer produces this diagnostic when a pattern variable is assigned a value inside a guard (when) clause.

The following code produces this diagnostic because the variable a is assigned a value inside the guard clause:

If there's a value you need to capture, then assign it to a different variable:

If there isn't a value you need to capture, then remove the assignment:

Keys in the platforms field can't have values.

The analyzer produces this diagnostic when a key in the platforms map has a value. To learn more about specifying your package's supported platforms, check out the documentation on platform declarations.

The following pubspec.yaml produces this diagnostic because the key web has a value.

Omit the value and leave the key without a value:

Values for keys in the platforms field are currently reserved for potential future behavior.

Object patterns can only use named fields.

The analyzer produces this diagnostic when an object pattern contains a field without specifying the getter name. Object pattern fields match against values that the object's getters return. Without a getter name specified, the pattern field can't access a value to attempt to match against.

The following code produces this diagnostic because the object pattern String(1) doesn't specify which getter of String to access and compare with the value 1:

Add the getter name to access the value, followed by a colon before the pattern to match against:

Positional super parameters can't be used when the super constructor invocation has a positional argument.

The analyzer produces this diagnostic when some, but not all, of the positional parameters provided to the constructor of the superclass are using a super parameter.

Positional super parameters are associated with positional parameters in the super constructor by their index. That is, the first super parameter is associated with the first positional parameter in the super constructor, the second with the second, and so on. The same is true for positional arguments. Having both positional super parameters and positional arguments means that there are two values associated with the same parameter in the superclass's constructor, and hence isn't allowed.

The following code produces this diagnostic because the constructor B.new is using a super parameter to pass one of the required positional parameters to the super constructor in A, but is explicitly passing the other in the super constructor invocation:

If all the positional parameters can be super parameters, then convert the normal positional parameters to be super parameters:

If some positional parameters can't be super parameters, then convert the super parameters to be normal parameters:

The name '{0}' is already used as an import prefix and can't be used to name a top-level element.

The analyzer produces this diagnostic when a name is used as both an import prefix and the name of a top-level declaration in the same library.

The following code produces this diagnostic because f is used as both an import prefix and the name of a function:

If you want to use the name for the import prefix, then rename the top-level declaration:

If you want to use the name for the top-level declaration, then rename the import prefix:

The name '{0}' refers to an import prefix, so it must be followed by '.'.

The analyzer produces this diagnostic when an import prefix is used by itself, without accessing any of the names declared in the libraries associated with the prefix. Prefixes aren't variables, and therefore can't be used as a value.

The following code produces this diagnostic because the prefix math is being used as if it were a variable:

If the code is incomplete, then reference something in one of the libraries associated with the prefix:

If the name is wrong, then correct the name.

The prefix '{0}' can't be used here because it's shadowed by a local declaration.

The analyzer produces this diagnostic when an import prefix is used in a context where it isn't visible because it was shadowed by a local declaration.

The following code produces this diagnostic because the prefix a is being used to access the class Future, but isn't visible because it's shadowed by the parameter a:

Rename either the prefix:

Or rename the local variable:

The private name '{0}', defined by '{1}', conflicts with the same name defined by '{2}'.

The analyzer produces this diagnostic when two mixins that define the same private member are used together in a single class in a library other than the one that defines the mixins.

Given a file a.dart containing the following code:

The following code produces this diagnostic because the mixins A and B both define the method _foo:

If you don't need both of the mixins, then remove one of them from the with clause:

If you need both of the mixins, then rename the conflicting member in one of the two mixins.

Named parameters can't start with an underscore.

The analyzer produces this diagnostic when the name of a named parameter starts with an underscore.

The following code produces this diagnostic because the named parameter _x starts with an underscore:

Rename the parameter so that it doesn't start with an underscore:

The setter '{0}' is private and can't be accessed outside the library that declares it.

The analyzer produces this diagnostic when a private setter is used in a library where it isn't visible.

Given a file a.dart that contains the following:

The following code produces this diagnostic because it references the private setter _f even though the setter isn't visible:

If you're able to make the setter public, then do so:

If you aren't able to make the setter public, then find a different way to implement the code.

The final variable '{0}' can't be read because it's potentially unassigned at this point.

The analyzer produces this diagnostic when a final local variable that isn't initialized at the declaration site is read at a point where the compiler can't prove that the variable is always initialized before it's referenced.

The following code produces this diagnostic because the final local variable x is read (on line 3) when it's possible that it hasn't yet been initialized:

Ensure that the variable has been initialized before it's read:

A record literal with exactly one positional field requires a trailing comma.

The analyzer produces this diagnostic when a record literal with a single positional field doesn't have a trailing comma after the field.

In some locations a record literal with a single positional field could also be a parenthesized expression. A trailing comma is required to disambiguate these two valid interpretations.

The following code produces this diagnostic because the record literal has one positional field but doesn't have a trailing comma:

Add a trailing comma:

A record type with exactly one positional field requires a trailing comma.

The analyzer produces this diagnostic when a record type annotation with a single positional field doesn't have a trailing comma after the field.

In some locations a record type with a single positional field could also be a parenthesized expression. A trailing comma is required to disambiguate these two valid interpretations.

The following code produces this diagnostic because the record type has one positional field, but doesn't have a trailing comma:

Add a trailing comma:

The compile-time constant expression depends on itself.

The analyzer produces this diagnostic when the value of a compile-time constant is defined in terms of itself, either directly or indirectly, creating an infinite loop.

The following code produces this diagnostic twice because both of the constants are defined in terms of the other:

Break the cycle by finding an alternative way of defining at least one of the constants:

Constructors can't redirect to themselves either directly or indirectly.

The analyzer produces this diagnostic when a constructor redirects to itself, either directly or indirectly, creating an infinite loop.

The following code produces this diagnostic because the generative constructors C.a and C.b each redirect to the other:

The following code produces this diagnostic because the factory constructors A and B each redirect to the other:

In the case of generative constructors, break the cycle by finding defining at least one of the constructors to not redirect to another constructor:

In the case of factory constructors, break the cycle by defining at least one of the factory constructors to do one of the following:

• Redirect to a generative constructor:

• Not redirect to another constructor:

• Not be a factory constructor:

'{0}' can't be a superinterface of itself: {1}.

'{0}' can't extend itself.

'{0}' can't implement itself.

'{0}' can't use itself as a mixin.

'{0}' can't use itself as a superclass constraint.

The analyzer produces this diagnostic when there's a circularity in the type hierarchy. This happens when a type, either directly or indirectly, is declared to be a subtype of itself.

The following code produces this diagnostic because the class A is declared to be a subtype of B, and B is a subtype of A:

Change the type hierarchy so that there's no circularity.

The {0} doesn't redeclare a {0} declared in a superinterface.

The analyzer produces this diagnostic when a member of an extension type is annotated with @redeclare, but none of the implemented interfaces has a member with the same name.

The following code produces this diagnostic because the member n declared by the extension type E is annotated with @redeclare, but C doesn't have a member named n:

If the annotated member has the right name, then remove the annotation:

If the annotated member is suppose to replace a member from the implemented interfaces, then change the name of the annotated member to match the member being replaced:

The constructor '{0}' couldn't be found in '{1}'.

The analyzer produces this diagnostic when a generative constructor redirects to a constructor that isn't defined.

The following code produces this diagnostic because the constructor C.a redirects to the constructor C.b, but C.b isn't defined:

If the missing constructor must be called, then define it:

If the missing constructor doesn't need to be called, then remove the redirect:

Generative constructors can't redirect to a factory constructor.

The analyzer produces this diagnostic when a generative constructor redirects to a factory constructor.

The following code produces this diagnostic because the generative constructor C.a redirects to the factory constructor C.b:

If the generative constructor doesn't need to redirect to another constructor, then remove the redirect.

If the generative constructor must redirect to another constructor, then make the other constructor be a generative (non-factory) constructor:

The redirecting constructor '{0}' can't redirect to a constructor of the abstract class '{1}'.

The analyzer produces this diagnostic when a constructor redirects to a constructor in an abstract class.

The following code produces this diagnostic because the factory constructor in A redirects to a constructor in B, but B is an abstract class:

If the code redirects to the correct constructor, then change the class so that it isn't abstract:

Otherwise, change the factory constructor so that it either redirects to a constructor in a concrete class, or has a concrete implementation.

The redirected constructor '{0}' has incompatible parameters with '{1}'.

The analyzer produces this diagnostic when a factory constructor attempts to redirect to another constructor, but the two have incompatible parameters. The parameters are compatible if all of the parameters of the redirecting constructor can be passed to the other constructor and if the other constructor doesn't require any parameters that aren't declared by the redirecting constructor.

The following code produces this diagnostic because the constructor for A doesn't declare a parameter that the constructor for B requires:

The following code produces this diagnostic because the constructor for A declares a named parameter (y) that the constructor for B doesn't allow:

If there's a different constructor that is compatible with the redirecting constructor, then redirect to that constructor:

Otherwise, update the redirecting constructor to be compatible:

The return type '{0}' of the redirected constructor isn't a subtype of '{1}'.

The analyzer produces this diagnostic when a factory constructor redirects to a constructor whose return type isn't a subtype of the type that the factory constructor is declared to produce.

The following code produces this diagnostic because A isn't a subclass of C, which means that the value returned by the constructor A() couldn't be returned from the constructor C():

If the factory constructor is redirecting to a constructor in the wrong class, then update the factory constructor to redirect to the correct constructor:

If the class defining the constructor being redirected to is the class that should be returned, then make it a subtype of the factory's return type:

The constructor '{0}' couldn't be found in '{1}'.

The analyzer produces this diagnostic when a constructor redirects to a constructor that doesn't exist.

The following code produces this diagnostic because the factory constructor in A redirects to a constructor in B that doesn't exist:

If the constructor being redirected to is correct, then define the constructor:

If a different constructor should be invoked, then update the redirect:

The name '{0}' isn't a type and can't be used in a redirected constructor.

One way to implement a factory constructor is to redirect to another constructor by referencing the name of the constructor. The analyzer produces this diagnostic when the redirect is to something other than a constructor.

The following code produces this diagnostic because f is a function:

If the constructor isn't defined, then either define it or replace it with a constructor that is defined.

If the constructor is defined but the class that defines it isn't visible, then you probably need to add an import.

If you're trying to return the value returned by a function, then rewrite the constructor to return the value from the constructor's body:

A constant redirecting constructor can't redirect to a non-constant constructor.

The analyzer produces this diagnostic when a constructor marked as const redirects to a constructor that isn't marked as const.

The following code produces this diagnostic because the constructor C.a is marked as const but redirects to the constructor C.b, which isn't:

If the non-constant constructor can be marked as const, then mark it as const:

If the non-constant constructor can't be marked as const, then either remove the redirect or remove const from the redirecting constructor:

A redirecting constructor can't redirect to a type alias that expands to a type parameter.

The analyzer produces this diagnostic when a redirecting factory constructor redirects to a type alias, and the type alias expands to one of the type parameters of the type alias. This isn't allowed because the value of the type parameter is a type rather than a class.

The following code produces this diagnostic because the redirect to B<A> is to a type alias whose value is T, even though it looks like the value should be A:

Use either a class name or a type alias that is defined to be a class rather than a type alias defined to be a type parameter:

Local variable '{0}' can't be referenced before it is declared.

The analyzer produces this diagnostic when a variable is referenced before it's declared. In Dart, variables are visible everywhere in the block in which they are declared, but can only be referenced after they are declared.

The analyzer also produces a context message that indicates where the declaration is located.

The following code produces this diagnostic because i is used before it is declared:

If you intended to reference the local variable, move the declaration before the first reference:

If you intended to reference a name from an outer scope, such as a parameter, instance field or top-level variable, then rename the local declaration so that it doesn't hide the outer variable.

Refutable patterns can't be used in an irrefutable context.

The analyzer produces this diagnostic when a refutable pattern is used in a context where only an irrefutable pattern is allowed.

The refutable patterns that are disallowed are:

• logical-or
• relational
• null-check
• constant

The contexts that are checked are:

• pattern-based variable declarations
• pattern-based for loops
• assignments with a pattern on the left-hand side

The following code produces this diagnostic because the null-check pattern, which is a refutable pattern, is in a pattern-based variable declaration, which doesn't allow refutable patterns:

Rewrite the code to not use a refutable pattern in an irrefutable context.

The constant expression type '{0}' is not assignable to the parameter type '{1}' of the '{2}' operator.

The analyzer produces this diagnostic when the operand of a relational pattern has a type that isn't assignable to the parameter of the operator that will be invoked.

The following code produces this diagnostic because the operand in the relational pattern (0) is an int, but the > operator defined in C expects an object of type C:

If the switch is using the correct value, then change the case to compare the value to the right type of object:

If the switch is using the wrong value, then change the expression used to compute the value being matched:

The return type of operators used in relational patterns must be assignable to 'bool'.

The analyzer produces this diagnostic when a relational pattern references an operator that doesn't produce a value of type bool.

The following code produces this diagnostic because the operator >, used in the relational pattern > c2, returns a value of type int rather than a bool:

If there's a different operator that should be used, then change the operator:

If the operator is expected to return bool, then update the declaration of the operator:

A map pattern can't contain a rest pattern.

The analyzer produces this diagnostic when a map pattern contains a rest pattern. Map patterns match a map with more keys than those explicitly given in the pattern (as long as the given keys match), so a rest pattern is unnecessary.

The following code produces this diagnostic because the map pattern contains a rest pattern:

Remove the rest pattern:

A rethrow must be inside of a catch clause.

The analyzer produces this diagnostic when a rethrow statement is outside a catch clause. The rethrow statement is used to throw a caught exception again, but there's no caught exception outside of a catch clause.

The following code produces this diagnostic because therethrow statement is outside of a catch clause:

If you're trying to rethrow an exception, then wrap the rethrow statement in a catch clause:

If you're trying to throw a new exception, then replace the rethrow statement with a throw expression:

Constructors can't return values.

The analyzer produces this diagnostic when a generative constructor contains a return statement that specifies a value to be returned. Generative constructors always return the object that was created, and therefore can't return a different object.

The following code produces this diagnostic because the return statement has an expression:

If the constructor should create a new instance, then remove either the return statement or the expression:

If the constructor shouldn't create a new instance, then convert it to be a factory constructor:

Can't return a value from a generator function that uses the 'async*' or 'sync*' modifier.

The analyzer produces this diagnostic when a generator function (one whose body is marked with either async* or sync*) uses either a return statement to return a value or implicitly returns a value because of using =>. In any of these cases, they should use yield instead of return.

The following code produces this diagnostic because the method f is a generator and is using return to return a value:

The following code produces this diagnostic because the function f is a generator and is implicitly returning a value:

If the function is using => for the body of the function, then convert it to a block function body, and use yield to return a value:

If the method is intended to be a generator, then use yield to return a value:

If the method isn't intended to be a generator, then remove the modifier from the body (or use async if you're returning a future):

'{0}' is annotated with 'doNotStore' and shouldn't be returned unless '{1}' is also annotated.

The analyzer produces this diagnostic when a value that is annotated with the doNotStore annotation is returned from a method, getter, or function that doesn't have the same annotation.

The following code produces this diagnostic because the result of invoking f shouldn't be stored, but the function g isn't annotated to preserve that semantic:

If the value that shouldn't be stored is the correct value to return, then mark the function with the doNotStore annotation:

Otherwise, return a different value from the function:

A value of type '{0}' can't be returned from the constructor '{2}' because it has a return type of '{1}'.

A value of type '{0}' can't be returned from the function '{2}' because it has a return type of '{1}'.

A value of type '{0}' can't be returned from the method '{2}' because it has a return type of '{1}'.

The analyzer produces this diagnostic when a method or function returns a value whose type isn't assignable to the declared return type.

The following code produces this diagnostic because f has a return type of String but is returning an int:

If the return type is correct, then replace the value being returned with a value of the correct type, possibly by converting the existing value:

If the value is correct, then change the return type to match:

The returned type '{0}' isn't returnable from a '{1}' function, as required by the closure's context.

The analyzer produces this diagnostic when the static type of a returned expression isn't assignable to the return type that the closure is required to have.

The following code produces this diagnostic because f is defined to be a function that returns a String, but the closure assigned to it returns an int:

If the return type is correct, then replace the returned value with a value of the correct type, possibly by converting the existing value:

The return value is missing after 'return'.

The analyzer produces this diagnostic when it finds a return statement without an expression in a function that declares a return type.

The following code produces this diagnostic because the function f is expected to return an int, but no value is being returned:

Add an expression that computes the value to be returned:

The class '{0}' wasn't exported from 'dart:core' until version 2.1, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when either the class Future or Stream is referenced in a library that doesn't import dart:async in code that has an SDK constraint whose lower bound is less than 2.1.0. In earlier versions, these classes weren't defined in dart:core, so the import was necessary.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.1.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the classes to be referenced:

If you need to support older versions of the SDK, then import the dart:async library.

The use of an as expression in a constant expression wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when an as expression inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using an as expression in a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the expression to be used:

If you need to support older versions of the SDK, then either rewrite the code to not use an as expression, or change the code so that the as expression isn't in a constant context:

The use of the operator '{0}' for 'bool' operands in a constant context wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when any use of the &, |, or ^ operators on the class bool inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using these operators in a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operators to be used:

If you need to support older versions of the SDK, then either rewrite the code to not use these operators, or change the code so that the expression isn't in a constant context:

Tearing off a constructor requires the 'constructor-tearoffs' language feature.

The analyzer produces this diagnostic when a constructor tear-off is found in code that has an SDK constraint whose lower bound is less than 2.15. Constructor tear-offs weren't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.15:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:

If you need to support older versions of the SDK, then rewrite the code to not use constructor tear-offs:

Using the operator '==' for non-primitive types wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when the operator == is used on a non-primitive type inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using this operator in a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:

If you need to support older versions of the SDK, then either rewrite the code to not use the == operator, or change the code so that the expression isn't in a constant context:

Extension methods weren't supported until version 2.6.0, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when an extension declaration or an extension override is found in code that has an SDK constraint whose lower bound is less than 2.6.0. Using extensions wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.6.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:

If you need to support older versions of the SDK, then rewrite the code to not make use of extensions. The most common way to do this is to rewrite the members of the extension as top-level functions (or methods) that take the value that would have been bound to this as a parameter:

The operator '>>>' wasn't supported until version 2.14.0, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when the operator >>> is used in code that has an SDK constraint whose lower bound is less than 2.14.0. This operator wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.14.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:

If you need to support older versions of the SDK, then rewrite the code to not use the >>> operator:

The use of an is expression in a constant context wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when an is expression inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using an is expression in a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the expression to be used:

If you need to support older versions of the SDK, then either rewrite the code to not use the is operator, or, if that isn't possible, change the code so that the is expression isn't in a constant context:

The type 'Never' wasn't supported until version 2.12.0, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when a reference to the class Never is found in code that has an SDK constraint whose lower bound is less than 2.12.0. This class wasn't defined in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.12.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the type to be used:

If you need to support older versions of the SDK, then rewrite the code to not reference this class:

Set literals weren't supported until version 2.2, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when a set literal is found in code that has an SDK constraint whose lower bound is less than 2.2.0. Set literals weren't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.2.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:

If you do need to support older versions of the SDK, then replace the set literal with code that creates the set without the use of a literal:

The for, if, and spread elements weren't supported until version 2.3.0, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when a for, if, or spread element is found in code that has an SDK constraint whose lower bound is less than 2.3.0. Using a for, if, or spread element wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:

If you need to support older versions of the SDK, then rewrite the code to not make use of those elements:

The if and spread elements weren't supported in constant expressions until version 2.5.0, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when an if or spread element inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.5.0. Using an if or spread element inside a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.5.0:

In the package that has that pubspec, code like the following produces this diagnostic:

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:

If you need to support older versions of the SDK, then rewrite the code to not make use of those elements:

If that isn't possible, change the code so that the element isn't in a constant context:

The element type '{0}' can't be assigned to the set type '{1}'.

The analyzer produces this diagnostic when an element in a set literal has a type that isn't assignable to the element type of the set.

The following code produces this diagnostic because the type of the string literal '0' is String, which isn't assignable to int, the element type of the set:

If the element type of the set literal is wrong, then change the element type of the set:

If the type of the element is wrong, then change the element:

The prefix of a deferred import can't be used in other import directives.

The analyzer produces this diagnostic when a prefix in a deferred import is also used as a prefix in other imports (whether deferred or not). The prefix in a deferred import can't be shared with other imports because the prefix is used to load the imported library.

The following code produces this diagnostic because the prefix x is used as the prefix for a deferred import and is also used for one other import:

If you can use a different name for the deferred import, then do so:

If you can use a different name for the other imports, then do so:

'Array's must have an 'Array' annotation that matches the dimensions.

The analyzer produces this diagnostic when the number of dimensions specified in an Array annotation doesn't match the number of nested arrays specified by the type of a field.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field a0 has a type with three nested arrays, but only two dimensions are given in the Array annotation:

If the type of the field is correct, then fix the annotation to have the required number of dimensions:

If the type of the field is wrong, then fix the type of the field:

Instance member '{0}' can't be accessed using static access.

The analyzer produces this diagnostic when a class name is used to access an instance field. Instance fields don't exist on a class; they exist only on an instance of the class.

The following code produces this diagnostic because x is an instance field:

If you intend to access a static field, then change the name of the field to an existing static field:

If you intend to access the instance field, then use an instance of the class to access the field:

The mixin '{0}' must be 'base' because the supertype '{1}' is 'base'.

The mixin '{0}' must be 'base' because the supertype '{1}' is 'final'.

The type '{0}' must be 'base', 'final' or 'sealed' because the supertype '{1}' is 'base'.

The type '{0}' must be 'base', 'final' or 'sealed' because the supertype '{1}' is 'final'.

The analyzer produces this diagnostic when a class or mixin has a direct or indirect supertype that is either base or final, but the class or mixin itself isn't marked either base, final, or sealed.

The following code produces this diagnostic because the class B is a subtype of A, and A is a base class, but B is neither base, final or sealed:

Add either base, final or sealed to the class or mixin declaration:

Classes and mixins can't implement deferred classes.

Classes can't extend deferred classes.

Classes can't mixin deferred classes.

The analyzer produces this diagnostic when a type (class or mixin) is a subtype of a class from a library being imported using a deferred import. The supertypes of a type must be compiled at the same time as the type, and classes from deferred libraries aren't compiled until the library is loaded.

For more information, check out Lazily loading a library.

Given a file a.dart that defines the class A:

The following code produces this diagnostic because the superclass of B is declared in a deferred library:

If you need to create a subtype of a type from the deferred library, then remove the deferred keyword:

'{0}' can't be used as a superclass constraint.

Classes and mixins can't implement '{0}'.

Classes can't extend '{0}'.

Classes can't mixin '{0}'.

The analyzer produces this diagnostic when one of the restricted classes is used in either an extends, implements, with, or on clause. The classes bool, double, FutureOr, int, Null, num, and String are all restricted in this way, to allow for more efficient implementations.

The following code produces this diagnostic because String is used in an extends clause:

The following code produces this diagnostic because String is used in an implements clause:

The following code produces this diagnostic because String is used in a with clause:

The following code produces this diagnostic because String is used in an on clause:

If a different type should be specified, then replace the type:

If there isn't a different type that would be appropriate, then remove the type, and possibly the whole clause:

The class '{0}' can't extend '{1}'.

The class '{0}' can't implement '{1}'.

The class '{0}' can't mix in '{1}'.

The analyzer produces this diagnostic when a class extends any FFI class other than Struct or Union, or implements or mixes in any FFI class. Struct and Union are the only FFI classes that can be subtyped, and then only by extending them.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C extends Double:

If the class should extend either Struct or Union, then change the declaration of the class:

If the class shouldn't extend either Struct or Union, then remove any references to FFI classes:

The class '{0}' shouldn't be extended, mixed in, or implemented because it's sealed.

The analyzer produces this diagnostic when a sealed class (one that either has the sealed annotation or inherits or mixes in a sealed class) is referenced in either the extends, implements, or with clause of a class or mixin declaration if the declaration isn't in the same package as the sealed class.

Given a library in a package other than the package being analyzed that contains the following:

The following code produces this diagnostic because C, which isn't in the same package as B, is extending the sealed class B:

If the class doesn't need to be a subtype of the sealed class, then change the declaration so that it isn't:

If the class needs to be a subtype of the sealed class, then either change the sealed class so that it's no longer sealed or move the subclass into the same package as the sealed class.

The class '{0}' can't extend '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The class '{0}' can't implement '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The class '{0}' can't mix in '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The analyzer produces this diagnostic when a class extends, implements, or mixes in a class that extends either Struct or Union. Classes can only extend either Struct or Union directly.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C extends S, and S extends Struct:

If you're trying to define a struct or union that shares some fields declared by a different struct or union, then extend Struct or Union directly and copy the shared fields:

A type alias that expands to a type parameter can't be implemented.

A type alias that expands to a type parameter can't be mixed in.

A type alias that expands to a type parameter can't be used as a superclass constraint.

A type alias that expands to a type parameter can't be used as a superclass.

The analyzer produces this diagnostic when a type alias that expands to a type parameter is used in an extends, implements, with, or on clause.

The following code produces this diagnostic because the type alias T, which expands to the type parameter S, is used in the extends clause of the class C:

Use the value of the type argument directly:

The type '{0}' of this parameter isn't a subtype of the type '{1}' of the associated super constructor parameter.

The analyzer produces this diagnostic when the type of a super parameter isn't a subtype of the corresponding parameter from the super constructor.

The following code produces this diagnostic because the type of the super parameter x in the constructor for B isn't a subtype of the parameter x in the constructor for A:

If the type of the super parameter can be the same as the parameter from the super constructor, then remove the type annotation from the super parameter (if the type is implicit, it is inferred from the type in the super constructor):

If the type of the super parameter can be a subtype of the corresponding parameter's type, then change the type of the super parameter:

If the type of the super parameter can't be changed, then use a normal parameter instead of a super parameter:

No associated named super constructor parameter.

The analyzer produces this diagnostic when there's a named super parameter in a constructor and the implicitly or explicitly invoked super constructor doesn't have a named parameter with the same name.

Named super parameters are associated by name with named parameters in the super constructor.

The following code produces this diagnostic because the constructor in A doesn't have a parameter named y:

If the super parameter should be associated with an existing parameter from the super constructor, then change the name to match the name of the corresponding parameter:

If the super parameter should be associated with a parameter that hasn't yet been added to the super constructor, then add it:

If the super parameter doesn't correspond to a named parameter from the super constructor, then change it to be a normal parameter:

No associated positional super constructor parameter.

The analyzer produces this diagnostic when there's a positional super parameter in a constructor and the implicitly or explicitly invoked super constructor doesn't have a positional parameter at the corresponding index.

Positional super parameters are associated with positional parameters in the super constructor by their index. That is, the first super parameter is associated with the first positional parameter in the super constructor, the second with the second, and so on.

The following code produces this diagnostic because the constructor in B has a positional super parameter, but there's no positional parameter in the super constructor in A:

The following code produces this diagnostic because the constructor in B has two positional super parameters, but there's only one positional parameter in the super constructor in A, which means that there's no corresponding parameter for y:

If the super constructor should have a positional parameter corresponding to the super parameter, then update the super constructor appropriately:

If the super constructor is correct, or can't be changed, then convert the super parameter into a normal parameter:

(Previously known as invalid_super_invocation)

The superconstructor call must be last in an initializer list: '{0}'.

The analyzer produces this diagnostic when the initializer list of a constructor contains an invocation of a constructor in the superclass, but the invocation isn't the last item in the initializer list.

The following code produces this diagnostic because the invocation of the superclass' constructor isn't the last item in the initializer list:

Move the invocation of the superclass' constructor to the end of the initializer list:

The enum constructor can't have a 'super' initializer.

The analyzer produces this diagnostic when the initializer list in a constructor in an enum contains an invocation of a super constructor.

The following code produces this diagnostic because the constructor in the enum E has a super constructor invocation in the initializer list:

Remove the super constructor invocation:

The 'super' keyword can't be used in an extension because an extension doesn't have a superclass.

The analyzer produces this diagnostic when a member declared inside an extension uses the super keyword . Extensions aren't classes and don't have superclasses, so the super keyword serves no purpose.

The following code produces this diagnostic because super can't be used in an extension:

Remove the super keyword :

The 'super' keyword can't be used in an extension type because an extension type doesn't have a superclass.

The analyzer produces this diagnostic when super is used in an instance member of an extension type. Extension types don't have superclasses, so there's no inherited member that could be invoked.

The following code produces this diagnostic because :

Replace or remove the super invocation:

Invalid context for 'super' invocation.

The analyzer produces this diagnostic when the keyword super is used outside of an instance method.

The following code produces this diagnostic because super is used in a top-level function:

Rewrite the code to not use super.

The redirecting constructor can't have a 'super' initializer.

The analyzer produces this diagnostic when a constructor that redirects to another constructor also attempts to invoke a constructor from the superclass. The superclass constructor will be invoked when the constructor that the redirecting constructor is redirected to is invoked.

The following code produces this diagnostic because the constructor C.a both redirects to C.b and invokes a constructor from the superclass:

Remove the invocation of the super constructor:

The 'case' shouldn't complete normally.

The analyzer produces this diagnostic when the statements following a case label in a switch statement could fall through to the next case or default label.

The following code produces this diagnostic because the case label with a value of zero (0) falls through to the default statements:

Change the flow of control so that the case won't fall through. There are several ways that this can be done, including adding one of the following at the end of the current list of statements:

• a return statement,
• a throw expression,
• a break statement,
• a continue, or
• an invocation of a function or method whose return type is Never.

Type '{0}' of the switch expression isn't assignable to the type '{1}' of case expressions.

The analyzer produces this diagnostic when the type of the expression in a switch statement isn't assignable to the type of the expressions in the case clauses.

The following code produces this diagnostic because the type of s (String) isn't assignable to the type of 0 (int):

If the type of the case expressions is correct, then change the expression in the switch statement to have the correct type:

If the type of the switch expression is correct, then change the case expressions to have the correct type:

A generative constructor of an abstract class can't be torn off.

The analyzer produces this diagnostic when a generative constructor from an abstract class is being torn off. This isn't allowed because it isn't valid to create an instance of an abstract class, which means that there isn't any valid use for the torn off constructor.

The following code produces this diagnostic because the constructor C.new is being torn off and the class C is an abstract class:

Tear off the constructor of a concrete class.

The Unicode code point 'U+{0}' changes the appearance of text from how it's interpreted by the compiler.

The analyzer produces this diagnostic when it encounters source that contains text direction Unicode code points. These code points cause source code in either a string literal or a comment to be interpreted and compiled differently than how it appears in editors, leading to possible security vulnerabilities.

The following code produces this diagnostic twice because there are hidden characters at the start and end of the label string:

If the code points are intended to be included in the string literal, then escape them:

If the code points aren't intended to be included in the string literal, then remove them:

The Unicode code point 'U+{0}' changes the appearance of text from how it's interpreted by the compiler.

The analyzer produces this diagnostic when it encounters source that contains text direction Unicode code points. These code points cause source code in either a string literal or a comment to be interpreted and compiled differently than how it appears in editors, leading to possible security vulnerabilities.

The following code produces this diagnostic twice because there are hidden characters at the start and end of the label string:

If the code points are intended to be included in the string literal, then escape them:

If the code points aren't intended to be included in the string literal, then remove them:

The type '{0}' of the thrown expression must be assignable to 'Object'.

The analyzer produces this diagnostic when the type of the expression in a throw expression isn't assignable to Object. It isn't valid to throw null, so it isn't valid to use an expression that might evaluate to null.

The following code produces this diagnostic because s might be null:

Add an explicit null-check to the expression:

The type of '{0}' can't be inferred because it depends on itself through the cycle: {1}.

The analyzer produces this diagnostic when a top-level variable has no type annotation and the variable's initializer refers to the variable, either directly or indirectly.

The following code produces this diagnostic because the variables x and y are defined in terms of each other, and neither has an explicit type, so the type of the other can't be inferred:

If the two variables don't need to refer to each other, then break the cycle:

If the two variables need to refer to each other, then give at least one of them an explicit type:

Note, however, that while this code doesn't produce any diagnostics, it will produce a stack overflow at runtime unless at least one of the variables is assigned a value that doesn't depend on the other variables before any of the variables in the cycle are referenced.

Typedefs can't reference themselves directly or recursively via another typedef.

The analyzer produces this diagnostic when a typedef refers to itself, either directly or indirectly.

The following code produces this diagnostic because F depends on itself indirectly through G:

Change one or more of the typedefs in the cycle so that none of them refer to themselves:

The deferred type '{0}' can't be used in a declaration, cast, or type test.

The analyzer produces this diagnostic when the type annotation is in a variable declaration, or the type used in a cast (as) or type test (is) is a type declared in a library that is imported using a deferred import. These types are required to be available at compile time, but aren't.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because the type of the parameter f is imported from a deferred library:

If you need to reference the imported type, then remove the deferred keyword:

If the import is required to be deferred and there's another type that is appropriate, then use that type in place of the type from the deferred library.

'{0}' doesn't conform to the bound '{2}' of the type parameter '{1}'.

The analyzer produces this diagnostic when a type argument isn't the same as or a subclass of the bounds of the corresponding type parameter.

The following code produces this diagnostic because String isn't a subclass of num:

Change the type argument to be a subclass of the bounds:

Tests for non-null should be done with '!= null'.

Tests for null should be done with '== null'.

The analyzer produces this diagnostic when there's a type check (using the as operator) where the type is Null. There's only one value whose type is Null, so the code is both more readable and more performant when it tests for null explicitly.

The following code produces this diagnostic because the code is testing to see whether the value of s is null by using a type check:

The following code produces this diagnostic because the code is testing to see whether the value of s is something other than null by using a type check:

Replace the type check with the equivalent comparison with null:

Static members can't reference type parameters of the class.

The analyzer produces this diagnostic when a static member references a type parameter that is declared for the class. Type parameters only have meaning for instances of the class.

The following code produces this diagnostic because the static method hasType has a reference to the type parameter T:

If the member can be an instance member, then remove the keyword static:

If the member must be a static member, then make the member be generic:

Note, however, that there isn't a relationship between T and S, so this second option changes the semantics from what was likely to be intended.

'{0}' can't be a supertype of its upper bound.

The analyzer produces this diagnostic when the bound of a type parameter (the type following the extends keyword) is either directly or indirectly the type parameter itself. Stating that the type parameter must be the same as itself or a subtype of itself or a subtype of itself isn't helpful because it will always be the same as itself.

The following code produces this diagnostic because the bound of T is T:

The following code produces this diagnostic because the bound of T1 is T2, and the bound of T2 is T1, effectively making the bound of T1 be T1:

If the type parameter needs to be a subclass of some type, then replace the bound with the required type:

If the type parameter can be any type, then remove the extends clause:

The name '{0}' isn't a type and can't be used in an 'is' expression.

The analyzer produces this diagnostic when the right-hand side of an is or is! test isn't a type.

The following code produces this diagnostic because the right-hand side is a parameter, not a type:

If you intended to use a type test, then replace the right-hand side with a type:

If you intended to use a different kind of test, then change the test:

The name '{0}' isn't defined, so it can't be used in an 'is' expression.

The analyzer produces this diagnostic when the name following the is in a type test expression isn't defined.

The following code produces this diagnostic because the name Srting isn't defined:

Replace the name with the name of a type:

A nullable expression can't be used as a condition.

A nullable expression can't be used as an iterator in a for-in loop.

A nullable expression can't be used in a spread.

A nullable expression can't be used in a yield-each statement.

The function can't be unconditionally invoked because it can be 'null'.

The method '{0}' can't be unconditionally invoked because the receiver can be 'null'.

The operator '{0}' can't be unconditionally invoked because the receiver can be 'null'.

The property '{0}' can't be unconditionally accessed because the receiver can be 'null'.

The analyzer produces this diagnostic when an expression whose type is potentially non-nullable is dereferenced without first verifying that the value isn't null.

The following code produces this diagnostic because s can be null at the point where it's referenced:

If the value really can be null, then add a test to ensure that members are only accessed when the value isn't null:

If the expression is a variable and the value should never be null, then change the type of the variable to be non-nullable:

If you believe that the value of the expression should never be null, but you can't change the type of the variable, and you're willing to risk having an exception thrown at runtime if you're wrong, then you can assert that the value isn't null:

Undefined name '{0}' used as an annotation.

The analyzer produces this diagnostic when a name that isn't defined is used as an annotation.

The following code produces this diagnostic because the name undefined isn't defined:

If the name is correct, but it isn't declared yet, then declare the name as a constant value:

If the name is wrong, replace the name with the name of a valid constant:

Otherwise, remove the annotation.

Undefined class '{0}'.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a class but either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because Piont isn't defined:

If the identifier isn't defined, then either define it or replace it with the name of a class that is defined. The example above can be corrected by fixing the spelling of the class:

If the class is defined but isn't visible, then you probably need to add an import.

The class '{0}' doesn't have a constructor named '{1}'.

The class '{0}' doesn't have an unnamed constructor.

The analyzer produces this diagnostic when a superclass constructor is invoked in the initializer list of a constructor, but the superclass doesn't define the constructor being invoked.

The following code produces this diagnostic because A doesn't have an unnamed constructor:

The following code produces this diagnostic because A doesn't have a constructor named m:

If the superclass defines a constructor that should be invoked, then change the constructor being invoked:

If the superclass doesn't define an appropriate constructor, then define the constructor being invoked:

There's no constant named '{0}' in '{1}'.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of an enum value, and the name either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because E doesn't define a constant named c:

If the constant should be defined, then add it to the declaration of the enum:

If the constant shouldn't be defined, then change the name to the name of an existing constant:

The enum doesn't have a constructor named '{0}'.

The enum doesn't have an unnamed constructor.

The analyzer produces this diagnostic when the constructor invoked to initialize an enum value doesn't exist.

The following code produces this diagnostic because the enum value c is being initialized by the unnamed constructor, but there's no unnamed constructor defined in E:

The following code produces this diagnostic because the enum value c is being initialized by the constructor named x, but there's no constructor named x defined in E:

If the enum value is being initialized by the unnamed constructor and one of the named constructors should have been used, then add the name of the constructor:

If the enum value is being initialized by the unnamed constructor and none of the named constructors are appropriate, then define the unnamed constructor:

If the enum value is being initialized by a named constructor and one of the existing constructors should have been used, then change the name of the constructor being invoked (or remove it if the unnamed constructor should be used):

If the enum value is being initialized by a named constructor and none of the existing constructors should have been used, then define a constructor with the name that was used:

The getter '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when an extension override is used to invoke a getter, but the getter isn't defined by the specified extension. The analyzer also produces this diagnostic when a static getter is referenced but isn't defined by the specified extension.

The following code produces this diagnostic because the extension E doesn't declare an instance getter named b:

The following code produces this diagnostic because the extension E doesn't declare a static getter named a:

If the name of the getter is incorrect, then change it to the name of an existing getter:

If the name of the getter is correct but the name of the extension is wrong, then change the name of the extension to the correct name:

If the name of the getter and extension are both correct, but the getter isn't defined, then define the getter:

The method '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when an extension override is used to invoke a method, but the method isn't defined by the specified extension. The analyzer also produces this diagnostic when a static method is referenced but isn't defined by the specified extension.

The following code produces this diagnostic because the extension E doesn't declare an instance method named b:

The following code produces this diagnostic because the extension E doesn't declare a static method named a:

If the name of the method is incorrect, then change it to the name of an existing method:

If the name of the method is correct, but the name of the extension is wrong, then change the name of the extension to the correct name:

If the name of the method and extension are both correct, but the method isn't defined, then define the method:

The operator '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when an operator is invoked on a specific extension when that extension doesn't implement the operator.

The following code produces this diagnostic because the extension E doesn't define the operator *:

If the extension is expected to implement the operator, then add an implementation of the operator to the extension:

If the operator is defined by a different extension, then change the name of the extension to the name of the one that defines the operator.

If the operator is defined on the argument of the extension override, then remove the extension override:

The setter '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when an extension override is used to invoke a setter, but the setter isn't defined by the specified extension. The analyzer also produces this diagnostic when a static setter is referenced but isn't defined by the specified extension.

The following code produces this diagnostic because the extension E doesn't declare an instance setter named b:

The following code produces this diagnostic because the extension E doesn't declare a static setter named a:

If the name of the setter is incorrect, then change it to the name of an existing setter:

If the name of the setter is correct, but the name of the extension is wrong, then change the name of the extension to the correct name:

If the name of the setter and extension are both correct, but the setter isn't defined, then define the setter:

The function '{0}' isn't defined.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a function but either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because the name emty isn't defined:

If the identifier isn't defined, then either define it or replace it with the name of a function that is defined. The example above can be corrected by fixing the spelling of the function:

If the function is defined but isn't visible, then you probably need to add an import or re-arrange your code to make the function visible.

The getter '{0}' isn't defined for the '{1}' function type.

The getter '{0}' isn't defined for the type '{1}'.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a getter but either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because String has no member named len:

If the identifier isn't defined, then either define it or replace it with the name of a getter that is defined. The example above can be corrected by fixing the spelling of the getter:

The library '{0}' doesn't export a member with the hidden name '{1}'.

The analyzer produces this diagnostic when a hide combinator includes a name that isn't defined by the library being imported.

The following code produces this diagnostic because dart:math doesn't define the name String:

If a different name should be hidden, then correct the name. Otherwise, remove the name from the list:

Undefined name '{0}'.

The analyzer produces this diagnostic when it encounters an identifier that either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because the name rihgt isn't defined:

If the identifier isn't defined, then either define it or replace it with an identifier that is defined. The example above can be corrected by fixing the spelling of the variable:

If the identifier is defined but isn't visible, then you probably need to add an import or re-arrange your code to make the identifier visible.

Undefined name 'await' in function body not marked with 'async'.

The analyzer produces this diagnostic when the name await is used in a method or function body without being declared, and the body isn't marked with the async keyword. The name await only introduces an await expression in an asynchronous function.

The following code produces this diagnostic because the name await is used in the body of f even though the body of f isn't marked with the async keyword:

Add the keyword async to the function body:

The method '{0}' isn't defined for the '{1}' function type.

The method '{0}' isn't defined for the type '{1}'.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a method but either isn't defined or isn't visible in the scope in which it's being referenced.

The following code produces this diagnostic because the identifier removeMiddle isn't defined:

If the identifier isn't defined, then either define it or replace it with the name of a method that is defined. The example above can be corrected by fixing the spelling of the method:

The named parameter '{0}' isn't defined.

The analyzer produces this diagnostic when a method or function invocation has a named argument, but the method or function being invoked doesn't define a parameter with the same name.

The following code produces this diagnostic because m doesn't declare a named parameter named a:

If the argument name is mistyped, then replace it with the correct name. The example above can be fixed by changing a to b:

If a subclass adds a parameter with the name in question, then cast the receiver to the subclass:

If the parameter should be added to the function, then add it:

The operator '{0}' isn't defined for the type '{1}'.

The analyzer produces this diagnostic when a user-definable operator is invoked on an object for which the operator isn't defined.

The following code produces this diagnostic because the class C doesn't define the operator +:

If the operator should be defined for the class, then define it:

The name '{0}' is being referenced through the prefix '{1}', but it isn't defined in any of the libraries imported using that prefix.

The analyzer produces this diagnostic when a prefixed identifier is found where the prefix is valid, but the identifier isn't declared in any of the libraries imported using that prefix.

The following code produces this diagnostic because dart:core doesn't define anything named a:

If the library in which the name is declared isn't imported yet, add an import for the library.

If the name is wrong, then change it to one of the names that's declared in the imported libraries.

The parameter '{0}' isn't defined by '{1}'.

The analyzer produces this diagnostic when an annotation of the form UseResult.unless(parameterDefined: parameterName) specifies a parameter name that isn't defined by the annotated function.

The following code produces this diagnostic because the function f doesn't have a parameter named b:

Change the argument named parameterDefined to match the name of one of the parameters to the function:

The setter '{0}' isn't defined for the '{1}' function type.

The setter '{0}' isn't defined for the type '{1}'.

The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a setter but either isn't defined or isn't visible in the scope in which the identifier is being referenced.

The following code produces this diagnostic because there isn't a setter named z:

If the identifier isn't defined, then either define it or replace it with the name of a setter that is defined. The example above can be corrected by fixing the spelling of the setter:

The library '{0}' doesn't export a member with the shown name '{1}'.

The analyzer produces this diagnostic when a show combinator includes a name that isn't defined by the library being imported.

The following code produces this diagnostic because dart:math doesn't define the name String:

If a different name should be shown, then correct the name. Otherwise, remove the name from the list:

(Previously known as undefined_super_method)

The getter '{0}' isn't defined in a superclass of '{1}'.

The method '{0}' isn't defined in a superclass of '{1}'.

The operator '{0}' isn't defined in a superclass of '{1}'.

The setter '{0}' isn't defined in a superclass of '{1}'.

The analyzer produces this diagnostic when an inherited member (method, getter, setter, or operator) is referenced using super, but there's no member with that name in the superclass chain.

The following code produces this diagnostic because Object doesn't define a method named n:

The following code produces this diagnostic because Object doesn't define a getter named g:

If the inherited member you intend to invoke has a different name, then make the name of the invoked member match the inherited member.

If the member you intend to invoke is defined in the same class, then remove the super..

If the member isn't defined, then either add the member to one of the superclasses or remove the invocation.

The platform '{0}' is not a recognized platform.

The analyzer produces this diagnostic when an unknown platform name is used as a key in the platforms map. To learn more about specifying your package's supported platforms, check out the documentation on platform declarations.

The following pubspec.yaml produces this diagnostic because the platform browser is unknown.

If you can rely on automatic platform detection, then omit the top-level platforms key.

If you need to manually specify the list of supported platforms, then write the platforms field as a map with known platform names as keys.

Unnecessary cast.

The analyzer produces this diagnostic when the value being cast is already known to be of the type that it's being cast to.

The following code produces this diagnostic because n is already known to be an int as a result of the is test:

Remove the unnecessary cast:

The dev dependency on {0} is unnecessary because there is also a normal dependency on that package.

The analyzer produces this diagnostic when there's an entry under dev_dependencies for a package that is also listed under dependencies. The packages under dependencies are available to all of the code in the package, so there's no need to also list them under dev_dependencies.

The following code produces this diagnostic because the package meta is listed under both dependencies and dev_dependencies:

Remove the entry under dev_dependencies (and the dev_dependencies key if that's the only package listed there):

The keyword 'final' isn't necessary because the parameter is implicitly 'final'.

The analyzer produces this diagnostic when either a field initializing parameter or a super parameter in a constructor has the keyword final. In both cases the keyword is unnecessary because the parameter is implicitly final.

The following code produces this diagnostic because the field initializing parameter has the keyword final:

The following code produces this diagnostic because the super parameter in B has the keyword final:

Remove the unnecessary final keyword:

The import of '{0}' is unnecessary because all of the used elements are also provided by the import of '{1}'.

The analyzer produces this diagnostic when an import isn't needed because all of the names that are imported and referenced within the importing library are also visible through another import.

Given a file a.dart that contains the following:

And, given a file b.dart that contains the following:

The following code produces this diagnostic because the class A, which is imported from a.dart, is also imported from b.dart. Removing the import of a.dart leaves the semantics unchanged:

If the import isn't needed, then remove it.

If some of the names imported by this import are intended to be used but aren't yet, and if those names aren't imported by other imports, then add the missing references to those names.

A double can't equal 'double.nan', so the condition is always 'false'.

A double can't equal 'double.nan', so the condition is always 'true'.

The analyzer produces this diagnostic when a value is compared to double.nan using either == or !=.

Dart follows the IEEE 754 floating-point standard for the semantics of floating point operations, which states that, for any floating point value x (including NaN, positive infinity, and negative infinity),

• NaN == x is always false
• NaN != x is always true

As a result, comparing any value to NaN is pointless because the result is already known (based on the comparison operator being used).

The following code produces this diagnostic because d is being compared to double.nan:

Use the getter double.isNaN instead:

The '!' will have no effect because the receiver can't be null.

The analyzer produces this diagnostic when the operand of the ! operator can't be null.

The following code produces this diagnostic because x can't be null:

Remove the null check operator (!):

Unnecessary 'noSuchMethod' declaration.

The analyzer produces this diagnostic when there's a declaration of noSuchMethod, the only thing the declaration does is invoke the overridden declaration, and the overridden declaration isn't the declaration in Object.

Overriding the implementation of Object's noSuchMethod (no matter what the implementation does) signals to the analyzer that it shouldn't flag any inherited abstract methods that aren't implemented in that class. This works even if the overriding implementation is inherited from a superclass, so there's no value to declare it again in a subclass.

The following code produces this diagnostic because the declaration of noSuchMethod in A makes the declaration of noSuchMethod in B unnecessary:

Remove the unnecessary declaration:

The null-assert pattern will have no effect because the matched type isn't nullable.

The analyzer produces this diagnostic when a null-assert pattern is used to match a value that isn't nullable.

The following code produces this diagnostic because the variable x isn't nullable:

Remove the null-assert pattern:

The null-check pattern will have no effect because the matched type isn't nullable.

The analyzer produces this diagnostic when a null-check pattern is used to match a value that isn't nullable.

The following code produces this diagnostic because the value x isn't nullable:

Remove the null-check pattern:

The operand can't be 'null', so the condition is always 'false'.

The operand can't be 'null', so the condition is always 'true'.

The operand must be 'null', so the condition is always 'false'.

The operand must be 'null', so the condition is always 'true'.

The analyzer produces this diagnostic when it finds an equality comparison (either == or !=) with one operand of null and the other operand can't be null. Such comparisons are always either true or false, so they serve no purpose.

The following code produces this diagnostic because x can never be null, so the comparison always evaluates to true:

The following code produces this diagnostic because x can never be null, so the comparison always evaluates to false:

If the other operand should be able to be null, then change the type of the operand:

If the other operand really can't be null, then remove the condition:

The '?' is unnecessary because '{0}' is nullable without it.

The analyzer produces this diagnostic when either the type dynamic or the type Null is followed by a question mark. Both of these types are inherently nullable so the question mark doesn't change the semantics.

The following code produces this diagnostic because the question mark following dynamic isn't necessary:

Remove the unneeded question mark:

Braces unnecessarily wrap this expression in a set literal.

The analyzer produces this diagnostic when a function that has a return type of void, Future<void>, or FutureOr<void> uses an expression function body (=>) and the returned value is a literal set containing a single element.

Although the language allows it, returning a value from a void function isn't useful because it can't be used at the call site. In this particular case the return is often due to a misunderstanding about the syntax. The braces aren't necessary and can be removed.

The following code produces this diagnostic because the closure being passed to g has a return type of void, but is returning a set:

Remove the braces from around the value:

Unnecessary type check; the result is always 'false'.

Unnecessary type check; the result is always 'true'.

The analyzer produces this diagnostic when the value of a type check (using either is or is!) is known at compile time.

The following code produces this diagnostic because the test a is Object? is always true:

If the type check doesn't check what you intended to check, then change the test:

If the type check does check what you intended to check, then replace the type check with its known value or completely remove it:

Static members from supertypes must be qualified by the name of the defining type.

The analyzer produces this diagnostic when code in one class references a static member in a superclass without prefixing the member's name with the name of the superclass. Static members can only be referenced without a prefix in the class in which they're declared.

The following code produces this diagnostic because the static field x is referenced in the getter g without prefixing it with the name of the defining class:

Prefix the name of the static member with the name of the declaring class:

Static members from the extended type or one of its superclasses must be qualified by the name of the defining type.

The analyzer produces this diagnostic when an undefined name is found, and the name is the same as a static member of the extended type or one of its superclasses.

The following code produces this diagnostic because m is a static member of the extended type C:

If you're trying to reference a static member that's declared outside the extension, then add the name of the class or extension before the reference to the member:

If you're referencing a member that isn't declared yet, add a declaration:

This case is covered by the previous cases.

The analyzer produces this diagnostic when a case clause in a switch statement doesn't match anything because all of the matchable values are matched by an earlier case clause.

The following code produces this diagnostic because the value 1 was matched in the preceding case:

Change one or both of the conflicting cases to match different values:

This default clause is covered by the previous cases.

The analyzer produces this diagnostic when a default clause in a switch statement doesn't match anything because all of the matchable values are matched by an earlier case clause.

The following code produces this diagnostic because the values E.e1 and E.e2 were matched in the preceding cases:

Remove the unnecessary default clause:

The exception variable '{0}' isn't used, so the 'catch' clause can be removed.

The analyzer produces this diagnostic when a catch clause is found, and neither the exception parameter nor the optional stack trace parameter are used in the catch block.

The following code produces this diagnostic because e isn't referenced:

Remove the unused catch clause:

The stack trace variable '{0}' isn't used and can be removed.

The analyzer produces this diagnostic when the stack trace parameter in a catch clause isn't referenced within the body of the catch block.

The following code produces this diagnostic because stackTrace isn't referenced:

If you need to reference the stack trace parameter, then add a reference to it. Otherwise, remove it:

The declaration '{0}' isn't referenced.

The analyzer produces this diagnostic when a private declaration isn't referenced in the library that contains the declaration. The following kinds of declarations are analyzed:

• Private top-level declarations and all of their members
• Private members of public declarations

Not all references to an element will mark it as "used":

• Assigning a value to a top-level variable (with a standard = assignment, or a null-aware ??= assignment) does not count as using it.
• Referring to an element in a doc comment reference does not count as using it.
• Referring to a class, mixin, or enum on the right side of an is expression does not count as using it.

Assuming that no code in the library references _C, the following code produces this diagnostic:

If the declaration isn't needed, then remove it.

If the declaration is intended to be used, then add the code to use it.

A value for optional parameter '{0}' isn't ever given.

The analyzer produces this diagnostic when a value is never passed for an optional parameter declared within a private declaration.

Assuming that no code in the library passes a value for y in any invocation of _m, the following code produces this diagnostic:

If the declaration isn't needed, then remove it:

If the declaration is intended to be used, then add the code to use it.

The value of the field '{0}' isn't used.

The analyzer produces this diagnostic when a private field is declared but never read, even if it's written in one or more places.

The following code produces this diagnostic because the field _originalValue isn't read anywhere in the library:

It might appear that the field _originalValue is being read in the initializer (_currentValue = _originalValue), but that is actually a reference to the parameter of the same name, not a reference to the field.

If the field isn't needed, then remove it.

If the field was intended to be used, then add the missing code.

Unused import: '{0}'.

The analyzer produces this diagnostic when an import isn't needed because none of the names that are imported are referenced within the importing library.

The following code produces this diagnostic because nothing defined in dart:async is referenced in the library:

If the import isn't needed, then remove it.

If some of the imported names are intended to be used, then add the missing code.

The label '{0}' isn't used.

The analyzer produces this diagnostic when a label that isn't used is found.

The following code produces this diagnostic because the label loop isn't referenced anywhere in the method:

If the label isn't needed, then remove it:

If the label is needed, then use it:

The value of the local variable '{0}' isn't used.

The analyzer produces this diagnostic when a local variable is declared but never read, even if it's written in one or more places.

The following code produces this diagnostic because the value of count is never read:

If the variable isn't needed, then remove it.

If the variable was intended to be used, then add the missing code.

'{0}' should be used. {1}.

The value of '{0}' should be used.

The analyzer produces this diagnostic when a function annotated with useResult is invoked, and the value returned by that function isn't used. The value is considered to be used if a member of the value is invoked, if the value is passed to another function, or if the value is assigned to a variable or field.

The following code produces this diagnostic because the invocation of c.a() isn't used, even though the method a is annotated with useResult:

If you intended to invoke the annotated function, then use the value that was returned:

If you intended to invoke a different function, then correct the name of the function being invoked:

The name {0} is shown, but isn't used.

The analyzer produces this diagnostic when a show combinator includes a name that isn't used within the library. Because it isn't referenced, the name can be removed.

The following code produces this diagnostic because the function max isn't used:

Either use the name or remove it:

Target of URI doesn't exist: '{0}'.

The analyzer produces this diagnostic when an import, export, or part directive is found where the URI refers to a file that doesn't exist.

If the file lib.dart doesn't exist, the following code produces this diagnostic:

If the URI was mistyped or invalid, then correct the URI.

If the URI is correct, then create the file.

Target of URI doesn't exist: '{0}'.

The analyzer produces this diagnostic when a doc-import is found where the URI refers to a file that doesn't exist.

If the file lib.dart doesn't exist, the following code produces this diagnostic:

If the URI was mistyped or invalid, then correct the URI.

If the URI is correct, then create the file.

Target of URI hasn't been generated: '{0}'.

The analyzer produces this diagnostic when an import, export, or part directive is found where the URI refers to a file that doesn't exist and the name of the file ends with a pattern that's commonly produced by code generators, such as one of the following:

• .g.dart
• .pb.dart
• .pbenum.dart
• .pbserver.dart
• .pbjson.dart
• .template.dart

If the file lib.g.dart doesn't exist, the following code produces this diagnostic:

If the file is a generated file, then run the generator that generates the file.

If the file isn't a generated file, then check the spelling of the URI or create the file.

URIs can't use string interpolation.

The analyzer produces this diagnostic when the string literal in an import, export, or part directive contains an interpolation. The resolution of the URIs in directives must happen before the declarations are compiled, so expressions can't be evaluated while determining the values of the URIs.

The following code produces this diagnostic because the string in the import directive contains an interpolation:

Remove the interpolation from the URI:

Dart native extensions are deprecated and aren't available in Dart 2.15.

The analyzer produces this diagnostic when a library is imported using the dart-ext scheme.

The following code produces this diagnostic because the native library x is being imported using a scheme of dart-ext:

Rewrite the code to use dart:ffi as a way of invoking the contents of the native library.

This expression has a type of 'void' so its value can't be used.

The analyzer produces this diagnostic when it finds an expression whose type is void, and the expression is used in a place where a value is expected, such as before a member access or on the right-hand side of an assignment.

The following code produces this diagnostic because f doesn't produce an object on which toString can be invoked:

Either rewrite the code so that the expression has a value or rewrite the code so that it doesn't depend on the value.

A member named 'values' can't be declared in an enum.

The analyzer produces this diagnostic when an enum declaration defines a member named values, whether the member is an enum value, an instance member, or a static member.

Any such member conflicts with the implicit declaration of the static getter named values that returns a list containing all the enum constants.

The following code produces this diagnostic because the enum E defines an instance member named values:

Change the name of the conflicting member:

Variable length 'Array's must only occur as the last field of Structs.

The analyzer produces this diagnostic when a variable length inline Array is not the last member of a Struct.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the field a0 has a type with three nested arrays, but only two dimensions are given in the Array annotation:

Move the variable length inline Array to be the last field in the struct.

If the inline array has a fixed size, annotate it with the size:

Variable patterns in declaration context can't specify 'var' or 'final' keyword.

The analyzer produces this diagnostic when a variable pattern is used within a declaration context.

The following code produces this diagnostic because the variable patterns in the record pattern are in a declaration context:

Remove the var or final keyword(s) within the variable pattern:

A value of type '{0}' can't be assigned to a const variable of type '{1}'.

The analyzer produces this diagnostic when the evaluation of a constant expression would result in a CastException.

The following code produces this diagnostic because the value of x is an int, which can't be assigned to y because an int isn't a String:

If the declaration of the constant is correct, then change the value being assigned to be of the correct type:

If the assigned value is correct, then change the declaration to have the correct type:

The value of the 'workspace' field is required to be a list of relative file paths.

The analyzer produces this diagnostic when the value of the workspace key isn't a list.

The following code produces this diagnostic because the value of the workspace key is a string when a list is expected:

Change the value of the workspace field so that it's a list:

Workspace entries are required to be directory paths (strings).

The analyzer produces this diagnostic when a workspace list contains a value that isn't a string.

The following code produces this diagnostic because the workspace list contains a map:

Change the workspace list so that it only contains valid POSIX-style directory paths:

Workspace values must be a relative path of a subdirectory of '{0}'.

The analyzer produces this diagnostic when a workspace list contains a value that is not a subdirectory of the directory containing the `pubspec.yaml`` file.

The following code produces this diagnostic because the value in the workspace list is not a relative path of a subdirectory of the directory containing the 'pubspec.yaml' file:

Change the workspace list so that it only contains only subdirectory paths.

Operator '-' should declare 0 or 1 parameter, but {0} found.

Operator '{0}' should declare exactly {1} parameters, but {2} found.

The analyzer produces this diagnostic when a declaration of an operator has the wrong number of parameters.

The following code produces this diagnostic because the operator + must have a single parameter corresponding to the right operand:

Add or remove parameters to match the required number:

Setters must declare exactly one required positional parameter.

The analyzer produces this diagnostic when a setter is found that doesn't declare exactly one required positional parameter.

The following code produces this diagnostic because the setter s declares two required parameters:

The following code produces this diagnostic because the setter s declares one optional parameter:

Change the declaration so that there's exactly one required positional parameter:

The type '{0}' is declared with {1} type parameters, but {2} type arguments were given.

The analyzer produces this diagnostic when a type that has type parameters is used and type arguments are provided, but the number of type arguments isn't the same as the number of type parameters.

The analyzer also produces this diagnostic when a constructor is invoked and the number of type arguments doesn't match the number of type parameters declared for the class.

The following code produces this diagnostic because C has one type parameter but two type arguments are provided when it is used as a type annotation:

The following code produces this diagnostic because C declares one type parameter, but two type arguments are provided when creating an instance:

Add or remove type arguments, as necessary, to match the number of type parameters defined for the type:

The constructor '{0}.{1}' doesn't have type parameters.

The analyzer produces this diagnostic when type arguments are provided after the name of a named constructor. Constructors can't declare type parameters, so invocations can only provide the type arguments associated with the class, and those type arguments are required to follow the name of the class rather than the name of the constructor.

The following code produces this diagnostic because the type parameters (<String>) follow the name of the constructor rather than the name of the class:

If the type arguments are for the class' type parameters, then move the type arguments to follow the class name:

If the type arguments aren't for the class' type parameters, then remove them:

The enum is declared with {0} type parameters, but {1} type arguments were given.

The analyzer produces this diagnostic when an enum value in an enum that has type parameters is instantiated and type arguments are provided, but the number of type arguments isn't the same as the number of type parameters.

The following code produces this diagnostic because the enum value c provides one type argument even though the enum E is declared to have two type parameters:

If the number of type parameters is correct, then change the number of type arguments to match the number of type parameters:

If the number of type arguments is correct, then change the number of type parameters to match the number of type arguments:

The extension '{0}' is declared with {1} type parameters, but {2} type arguments were given.

The analyzer produces this diagnostic when an extension that has type parameters is used and type arguments are provided, but the number of type arguments isn't the same as the number of type parameters.

The following code produces this diagnostic because the extension E is declared to have a single type parameter (T), but the extension override has two type arguments:

Change the type arguments so that there are the same number of type arguments as there are type parameters:

The method '{0}' is declared with {1} type parameters, but {2} type arguments are given.

The analyzer produces this diagnostic when a method or function is invoked with a different number of type arguments than the number of type parameters specified in its declaration. There must either be no type arguments or the number of arguments must match the number of parameters.

The following code produces this diagnostic because the invocation of the method m has two type arguments, but the declaration of m only has one type parameter:

If the type arguments are necessary, then make them match the number of type parameters by either adding or removing type arguments:

If the type arguments aren't necessary, then remove them:

Yield statements must be in a generator function (one marked with either 'async*' or 'sync*').

Yield-each statements must be in a generator function (one marked with either 'async*' or 'sync*').

The analyzer produces this diagnostic when a yield or yield* statement appears in a function whose body isn't marked with one of the async* or sync* modifiers.

The following code produces this diagnostic because yield is being used in a function whose body doesn't have a modifier:

The following code produces this diagnostic because yield* is being used in a function whose body has the async modifier rather than the async* modifier:

Add a modifier, or change the existing modifier to be either async* or sync*:

A yielded value of type '{0}' must be assignable to '{1}'.

The type '{0}' implied by the 'yield*' expression must be assignable to '{1}'.

The analyzer produces this diagnostic when the type of object produced by a yield or yield* expression doesn't match the type of objects that are to be returned from the Iterable or Stream types that are returned from a generator (a function or method marked with either sync* or async*).

The following code produces this diagnostic because the getter zero is declared to return an Iterable that returns integers, but the yield is returning a string from the iterable:

If the return type of the function is correct, then fix the expression following the keyword yield to return the correct type:

If the expression following the yield is correct, then change the return type of the function to allow it:

The function '{0}' should have a return type but doesn't.

The method '{0}' should have a return type but doesn't.

The analyzer produces this diagnostic when a method or function doesn't have an explicit return type.

The following code produces this diagnostic because the function f doesn't have a return type:

Add an explicit return type:

Statement should be on a separate line.

The analyzer produces this diagnostic when the code being controlled by a control flow statement (if, for, while, or do) is on the same line as the control flow statement.

The following code produces this diagnostic because the return statement is on the same line as the if that controls whether the return will be executed:

Put the controlled statement onto a separate, indented, line:

Required named parameters should be before optional named parameters.

The analyzer produces this diagnostic when required named parameters occur after optional named parameters.

The following code produces this diagnostic because the required parameter x is after the optional parameter y:

Reorder the parameters so that all required named parameters are before any optional named parameters:

Use 'package:' imports for files in the 'lib' directory.

The analyzer produces this diagnostic when an import in a library inside the lib directory uses a relative path to import another library inside the lib directory of the same package.

Given that a file named a.dart and the code below are both inside the lib directory of the same package, the following code produces this diagnostic because a relative URI is used to import a.dart:

Use a package import:

The member '{0}' overrides an inherited member but isn't annotated with '@override'.

The analyzer produces this diagnostic when a member overrides an inherited member, but isn't annotated with @override.

The following code produces this diagnostic because the method m in the class B overrides the method with the same name in class A, but isn't marked as an intentional override:

If the member in the subclass is intended to override the member in the superclass, then add an @override annotation:

If the member in the subclass is not intended to override the member in the superclass, then rename one of the members:

Method invocation or property access on a 'dynamic' target.

The analyzer produces this diagnostic when a member of a class is accessed on an expression whose type is dynamic.

The following code produces this diagnostic because the getter length is being invoked on s, which has the type dynamic:

Provide enough type information that the expression has a type other than dynamic:

Empty statements are not allowed in an 'else' clause.

The analyzer produces this diagnostic when the statement after an else is an empty statement (a semicolon).

For more information, see the documentation for avoid_empty_else.

The following code produces this diagnostic because the statement following the else is an empty statement:

If the statement after the empty statement is intended to be executed only when the condition is false, then remove the empty statement:

If there is no code that is intended to be executed only when the condition is false, then remove the whole else clause:

Function literals shouldn't be passed to 'forEach'.

The analyzer produces this diagnostic when the argument to Iterable.forEach is a closure.

The following code produces this diagnostic because the argument to the invocation of forEach is a closure:

If the closure can be replaced by a tear-off, then replace the closure:

If the closure can't be replaced by a tear-off, then use a for loop to iterate over the elements:

Don't use the type 'FutureOr'.

The analyzer produces this diagnostic when the type FutureOr<void> is used as the type of a result (to be precise: it is used in a position that isn't contravariant). The type FutureOr<void> is problematic because it may appear to encode that a result is either a Future<void>, or the result should be discarded (when it is void). However, there is no safe way to detect whether we have one or the other case because an expression of type void can evaluate to any object whatsoever, including a future of any type.

It is also conceptually unsound to have a type whose meaning is something like "ignore this object; also, take a look because it might be a future".

An exception is made for contravariant occurrences of the type FutureOr<void> (e.g., for the type of a formal parameter), and no warning is emitted for these occurrences. The reason for this exception is that the type does not describe a result, it describes a constraint on a value provided by others. Similarly, an exception is made for type alias declarations, because they may well be used in a contravariant position (e.g., as the type of a formal parameter). Hence, in type alias declarations, only the type parameter bounds are checked.

A replacement for the type FutureOr<void> which is often useful is Future<void>?. This type encodes that a result is either a Future<void> or it is null, and there is no ambiguity at run time since no object can have both types.

It may not always be possible to use the type Future<void>? as a replacement for the type FutureOr<void>, because the latter is a supertype of all types, and the former is not. In this case it may be a useful remedy to replace FutureOr<void> by the type void.

Redundant initialization to 'null'.

The analyzer produces this diagnostic when a nullable variable is explicitly initialized to null. The variable can be a local variable, field, or top-level variable.

A variable or field that isn't explicitly initialized automatically gets initialized to null. There's no concept of "uninitialized memory" in Dart.

The following code produces this diagnostic because the variable f is explicitly initialized to null:

Remove the unnecessary initialization:

Don't invoke 'print' in production code.

The analyzer produces this diagnostic when the function print is invoked in production code.

The following code produces this diagnostic because the function print can't be invoked in production:

If you're writing code that uses Flutter, then use the function debugPrint, guarded by a test using kDebugMode:

If you're writing code that doesn't use Flutter, then use a logging service, such as package:logging, to write the information.

Can't use a relative path to import a library in 'lib'.

The analyzer produces this diagnostic when the URI in an import directive has lib in the path.

Assuming that there is a file named a.dart in the lib directory:

The following code produces this diagnostic because the import contains a path that includes lib:

Rewrite the import to not include lib in the URI:

The parameter name '{0}' doesn't match the name '{1}' in the overridden method.

The analyzer produces this diagnostic when a method that overrides a method from a superclass changes the names of the parameters.

The following code produces this diagnostic because the parameter of the method m in B is named b, which is different from the name of the overridden method's parameter in A:

Rename one of the parameters so that they are the same:

Unnecessary return type on a setter.

The analyzer produces this diagnostic when a setter has an explicit return type.

Setters never return a value, so declaring the return type of one is redundant.

The following code produces this diagnostic because the setter s has an explicit return type (void):

Remove the return type:

Don't return 'null' from a function with a return type of 'void'.

Don't return 'null' from a method with a return type of 'void'.

The analyzer produces this diagnostic when a function that has a return type of void explicitly returns null.

The following code produces this diagnostic because there is an explicit return of null in a void function:

Remove the unnecessary explicit null:

The type parameter '{0}' shadows a type parameter from the enclosing {1}.

The analyzer produces this diagnostic when a type parameter shadows a type parameter from an enclosing declaration.

Shadowing a type parameter with a different type parameter can lead to subtle bugs that are difficult to debug.

The following code produces this diagnostic because the type parameter T defined by the method m shadows the type parameter T defined by the class C:

Rename one of the type parameters:

Unnecessary cascade expression.

The analyzer produces this diagnostic when a single cascade operator is used and the value of the expression isn't being used for anything (such as being assigned to a variable or being passed as an argument).

The following code produces this diagnostic because the value of the cascade expression s..length isn't being used:

Replace the cascade operator with a simple access operator:

Use of an async 'dart:io' method.

The analyzer produces this diagnostic when an asynchronous file I/O method with a synchronous equivalent is used.

The following are the specific flagged asynchronous methods:

• Directory.exists
• Directory.stat
• File.lastModified
• File.exists
• File.stat
• FileSystemEntity.isDirectory
• FileSystemEntity.isFile
• FileSystemEntity.isLink
• FileSystemEntity.type

The following code produces this diagnostic because the async method exists is invoked:

Use the synchronous version of the method:

Using 'toString' on a 'Type' is not safe in production code.

The analyzer produces this diagnostic when the method toString is invoked on a value whose static type is Type.

The following code produces this diagnostic because the method toString is invoked on the Type returned by runtimeType:

If it's essential that the type is exactly the same, then use an explicit comparison:

If it's alright for instances of subtypes of the type to return true, then use a type check:

The parameter name '{0}' matches a visible type name.

The analyzer produces this diagnostic when the name of a parameter in a parameter list is the same as a visible type (a type whose name is in scope).

This often indicates that the intended name of the parameter is missing, causing the name of the type to be used as the name of the parameter rather than the type of the parameter. Even when that's not the case (the name of the parameter is intentional), the name of the parameter will shadow the existing type, which can lead to bugs that are difficult to diagnose.

The following code produces this diagnostic because the function f has a parameter named int, which shadows the type int from dart:core:

If the parameter name is missing, then add a name for the parameter:

If the parameter is intended to have an implicit type of dynamic, then rename the parameter so that it doesn't shadow the name of any visible type:

Unnecessary instance of 'Container'.

The analyzer produces this diagnostic when a widget tree contains an instance of Container and the only argument to the constructor is child:.

The following code produces this diagnostic because the invocation of the Container constructor only has a child: argument:

If you intended to provide other arguments to the constructor, then add them:

If no other arguments are needed, then unwrap the child widget:

Don't use web-only libraries outside Flutter web plugins.

The analyzer produces this diagnostic when a library in a package that isn't a web plugin contains an import of a web-only library:

• dart:html
• dart:js
• dart:js_util
• dart:js_interop
• dart:js_interop_unsafe
• package:js
• package:web

When found in a package that isn't a web plugin, the following code produces this diagnostic because it imports dart:html:

If the package isn't intended to be a web plugin, then remove the import:

If the package is intended to be a web plugin, then add the following lines to the pubspec.yaml file of the package:

See Developing packages & plugins for more information.

Uses 'await' on an instance of '{0}', which is not a subtype of 'Future'.

The analyzer produces this diagnostic when the expression after await has any type other than Future<T>, FutureOr<T>, Future<T>?, FutureOr<T>? or dynamic.

An exception is made for the expression await null because it is a common way to introduce a microtask delay.

Unless the expression can produce a Future, the await is unnecessary and can cause a reader to assume a level of asynchrony that doesn't exist.

The following code produces this diagnostic because the expression after await has the type int:

Remove the await:

The extension name '{0}' isn't an UpperCamelCase identifier.

The analyzer produces this diagnostic when the name of an extension doesn't use the 'UpperCamelCase' naming convention.

The following code produces this diagnostic because the name of the extension doesn't start with an uppercase letter:

If the extension needs to have a name (needs to be visible outside this library), then rename the extension so that it has a valid name:

If the extension doesn't need to have a name, then remove the name of the extension:

The type name '{0}' isn't an UpperCamelCase identifier.

The analyzer produces this diagnostic when the name of a type (a class, mixin, enum, or typedef) doesn't use the 'UpperCamelCase' naming convention.

The following code produces this diagnostic because the name of the class doesn't start with an uppercase letter:

Rename the type so that it has a valid name:

Uncancelled instance of 'StreamSubscription'.

The analyzer produces this diagnostic when an instance of StreamSubscription is created but the method cancel isn't invoked.

The following code produces this diagnostic because the subscription isn't canceled:

Cancel the subscription:

Unclosed instance of 'Sink'.

The analyzer produces this diagnostic when an instance of Sink is created but the method close isn't invoked.

The following code produces this diagnostic because the sink isn't closed:

Close the sink:

The argument type '{0}' isn't related to '{1}'.

The analyzer produces this diagnostic when any one of several methods in the core libraries are invoked with arguments of an inappropriate type. These methods are ones that don't provide a specific enough type for the parameter to allow the normal type checking to catch the error.

The arguments that are checked are:

• an argument to Iterable<E>.contains should be related to E
• an argument to List<E>.remove should be related to E
• an argument to Map<K, V>.containsKey should be related to K
• an argument to Map<K, V>.containsValue should be related to V
• an argument to Map<K, V>.remove should be related to K
• an argument to Map<K, V>.[] should be related to K
• an argument to Queue<E>.remove should be related to E
• an argument to Set<E>.lookup should be related to E
• an argument to Set<E>.remove should be related to E

The following code produces this diagnostic because the argument to contains is a String, which isn't assignable to int, the element type of the list l:

If the element type is correct, then change the argument to have the same type:

If the argument type is correct, then change the element type:

The constant name '{0}' isn't a lowerCamelCase identifier.

The analyzer produces this diagnostic when the name of a constant doesn't follow the lowerCamelCase naming convention.

The following code produces this diagnostic because the name of the top-level variable isn't a lowerCamelCase identifier:

Rewrite the name to follow the lowerCamelCase naming convention:

Use of '{0}' in a 'finally' clause.

The analyzer produces this diagnostic when a finally clause contains a return, break, or continue statement.

The following code produces this diagnostic because there is a return statement inside a finally block:

If the statement isn't needed, then remove the statement, and remove the finally clause if the block is empty:

If the statement is needed, then move the statement outside the finally block:

Statements in {0} should be enclosed in a block.

The analyzer produces this diagnostic when a control structure (if, for, while, or do statement) has a statement other than a block.

The following code produces this diagnostic because the then statement is not enclosed in a block:

Add braces around the statement that should be a block:

Dangling library doc comment.

The analyzer produces this diagnostic when a documentation comment that appears to be library documentation isn't followed by a library directive. More specifically, it is produced when a documentation comment appears before the first directive in the library, assuming that it isn't a library directive, or before the first top-level declaration and is separated from the declaration by one or more blank lines.

The following code produces this diagnostic because there's a documentation comment before the first import directive:

The following code produces this diagnostic because there's a documentation comment before the first class declaration, but there's a blank line between the comment and the declaration.

If the comment is library documentation, then add a library directive without a name:

If the comment is documentation for the following declaration, then remove the blank line:

The imported package '{0}' isn't a dependency of the importing package.

The analyzer produces this diagnostic when a package import refers to a package that is not specified in the pubspec.yaml file.

Depending explicitly on packages that you reference ensures they will always exist and allows you to put a dependency constraint on them to guard against breaking changes.

Given a pubspec.yaml file containing the following:

The following code produces this diagnostic because there is no dependency on the package a:

Whether the dependency should be a regular dependency or dev dependency depends on whether the package is referenced from a public library (one under either lib or bin), or only private libraries, (such as one under test).

If the package is referenced from at least one public library, then add a regular dependency on the package to the pubspec.yaml file under the dependencies field:

If the package is referenced only from private libraries, then add a dev dependency on the package to the pubspec.yaml file under the dev_dependencies field:

The public property isn't described by either 'debugFillProperties' or 'debugDescribeChildren'.

The analyzer produces this diagnostic when a class that implements Diagnosticable has a public property that isn't added as a property in either a debugFillProperties or debugDescribeChildren method.

The following code produces this diagnostic because the property p2 isn't added in the debugFillProperties method:

If there isn't on override of either the debugFillProperties or debugDescribeChildren method, then add one.

Add a description of the property in the debugFillProperties or debugDescribeChildren method:

Empty catch block.

The analyzer produces this diagnostic when the block in a catch clause is empty.

The following code produces this diagnostic because the catch block is empty:

If the exception shouldn't be ignored, then add code to handle the exception:

If the exception is intended to be ignored, then add a comment explaining why:

If the exception is intended to be ignored and there isn't any good explanation for why, then rename the exception parameter:

Empty constructor bodies should be written using a ';' rather than '{}'.

The analyzer produces this diagnostic when a constructor has an empty block body.

The following code produces this diagnostic because the constructor for C has a block body that is empty:

Replace the block with a semicolon:

Unnecessary empty statement.

The analyzer produces this diagnostic when an empty statement is found.

The following code produces this diagnostic because the statement controlled by the while loop is an empty statement:

If there are no statements that need to be controlled, then remove both the empty statement and the control structure it's part of (being careful that any other code being removed doesn't have a side-effect that needs to be preserved):

If there are no statements that need to be controlled but the control structure is still required for other reasons, then replace the empty statement with a block to make the structure of the code more obvious:

If there are statements that need to be controlled, remove the empty statement and adjust the code so that the appropriate statements are being controlled, possibly adding a block:

The file name '{0}' isn't a lower_case_with_underscores identifier.

The analyzer produces this diagnostic when the name of a .dart file doesn't use lower_case_with_underscores.

A file named SliderMenu.dart produces this diagnostic because the file name uses the UpperCamelCase convention.

Rename the file to use the lower_case_with_underscores convention, such as slider_menu.dart.

Missing a corresponding override of '{0}'.

The analyzer produces this diagnostic when a class or mixin either overrides the definition of == but doesn't override the definition of hashCode, or conversely overrides the definition of hashCode but doesn't override the definition of ==.

Both the == operator and the hashCode property of objects must be consistent for a common hash map implementation to function properly. As a result, when overriding either method, both should be overridden.

The following code produces this diagnostic because the class C overrides the == operator but doesn't override the getter hashCode:

If you need to override one of the members, then add an override of the other:

If you don't need to override either of the members, then remove the unnecessary override:

Import of a library in the 'lib/src' directory of another package.

The analyzer produces this diagnostic when an import references a library that's inside the lib/src directory of a different package, which violates the convention for pub packages.

The following code, assuming that it isn't part of the ffi package, produces this diagnostic because the library being imported is inside the top-level src directory:

If the library being imported contains code that's part of the public API, then import the public library that exports the public API:

If the library being imported isn't part of the public API of the package, then either find a different way to accomplish your goal, assuming that it's possible, or open an issue asking the package authors to make it part of the public API.

Implicit tear-off of the 'call' method.

The analyzer produces this diagnostic when an object with a call method is assigned to a function-typed variable, implicitly tearing off the call method.

The following code produces this diagnostic because an instance of Callable is passed to a function expecting a Function:

Explicitly tear off the call method:

Cast from '{0}' to '{1}' casts a Dart value to a JS interop type, which might not be platform-consistent.

Cast from '{0}' to '{1}' casts a JS interop value to a Dart type, which might not be platform-consistent.

Cast from '{0}' to '{1}' casts a JS interop value to an incompatible JS interop type, which might not be platform-consistent.

Runtime check between '{0}' and '{1}' checks whether a Dart value is a JS interop type, which might not be platform-consistent.

Runtime check between '{0}' and '{1}' checks whether a JS interop value is a Dart type, which might not be platform-consistent.

Runtime check between '{0}' and '{1}' involves a non-trivial runtime check between two JS interop types that might not be platform-consistent.

Runtime check between '{0}' and '{1}' involves a runtime check between a JS interop value and an unrelated JS interop type that will always be true and won't check the underlying type.

The analyzer produces this diagnostic when an is test has either:

• a JS interop type on the right-hand side, whether directly or as a type argument to another type, or
• a JS interop value on the left-hand side.

The following code produces this diagnostic because the JS interop type JSBoolean is on the right-hand side of an is test:

The following code produces this diagnostic because the JS interop type JSString is used as a type argument on the right-hand side of an is test:

The following code produces this diagnostic because the JS interop value a is on the left-hand side of an is test:

Use a JS interop helper, such as isA, to check the underlying type of JS interop values:

Uses of '{0}' should not be submitted to source control.

The analyzer produces this diagnostic when a member that is annotated with @doNotSubmit is referenced outside of a member declaration that is also annotated with @doNotSubmit.

Given a file a.dart containing the following declaration:

The following code produces this diagnostic because the declaration is being referenced outside of a member that is also annotated with @doNotSubmit:

Most commonly, when complete with local testing, the reference to the member should be removed.

If building additional functionality on top of the member, annotate the newly added member with @doNotSubmit as well:

This annotation should be attached to a library directive.

The analyzer produces this diagnostic when an annotation that applies to a whole library isn't associated with a library directive.

The following code produces this diagnostic because the TestOn annotation, which applies to the whole library, is associated with an import directive rather than a library directive:

Associate the annotation with a library directive, adding one if necessary:

The library name '{0}' isn't a lower_case_with_underscores identifier.

The analyzer produces this diagnostic when the name of a library doesn't use the lower_case_with_underscores naming convention.

The following code produces this diagnostic because the library name libraryName isn't a lower_case_with_underscores identifier:

If the library name is not required, then remove the library name:

If the library name is required, then convert it to use the lower_case_with_underscores naming convention:

The prefix '{0}' isn't a lower_case_with_underscores identifier.

The analyzer produces this diagnostic when an import prefix doesn't use the lower_case_with_underscores naming convention.

The following code produces this diagnostic because the prefix ffiSupport isn't a lower_case_with_underscores identifier:

Convert the prefix to use the lower_case_with_underscores naming convention:

Invalid use of a private type in a public API.

The analyzer produces this diagnostic when a type that is not part of the public API of a library is referenced in the public API of that library.

Using a private type in a public API can make the API unusable outside the defining library.

The following code produces this diagnostic because the parameter c of the public function f has a type that is library private (_C):

If the API doesn't need to be used outside the defining library, then make it private:

If the API needs to be part of the public API of the library, then either use a different type that's public, or make the referenced type public:

The Boolean expression has a constant value.

The analyzer produces this diagnostic when the value of the condition in an if or loop statement is known to be either always true or always false. An exception is made for a while loop whose condition is the Boolean literal true.

The following code produces this diagnostic because the condition will always evaluate to true:

The lint will evaluate a subset of expressions that are composed of constants, so the following code will also produce this diagnostic because the condition will always evaluate to false:

If the condition is wrong, then correct the condition so that it's value can't be known at compile time:

If the condition is correct, then simplify the code to not evaluate the condition:

Missing whitespace between adjacent strings.

The analyzer produces this diagnostic for a pair of adjacent string literals unless either the left-hand string ends in whitespace or the right-hand string begins with whitespace.

The following code produces this diagnostic because neither the left nor the right string literal includes a space to separate the words that will be joined:

Add whitespace at the end of the left-hand literal or at the beginning of the right-hand literal:

Don't use adjacent strings in a list literal.

The analyzer produces this diagnostic when two string literals are adjacent in a list literal. Adjacent strings in Dart are concatenated together to form a single string, but the intent might be for each string to be a separate element in the list.

The following code produces this diagnostic because the strings 'a' and 'b' are adjacent:

If the two strings are intended to be separate elements of the list, then add a comma between them:

If the two strings are intended to be a single concatenated string, then either manually merge the strings:

Or use the + operator to concatenate the strings:

The value of the case clause ('{0}') is equal to the value of an earlier case clause ('{1}').

The analyzer produces this diagnostic when two or more case clauses in the same switch statement have the same value.

Any case clauses after the first can't be executed, so having duplicate case clauses is misleading.

This diagnostic is often the result of either a typo or a change to the value of a constant.

The following code produces this diagnostic because two case clauses have the same value (1):

If one of the clauses should have a different value, then change the value of the clause:

If the value is correct, then merge the statements into a single clause:

The library prefix '{0}' starts with an underscore.

The analyzer produces this diagnostic when the name of a prefix declared on an import starts with an underscore.

Library prefixes are inherently not visible outside the declaring library, so a leading underscore indicating private adds no value.

The following code produces this diagnostic because the prefix _core starts with an underscore:

Remove the underscore:

The local variable '{0}' starts with an underscore.

The analyzer produces this diagnostic when the name of a local variable starts with an underscore.

Local variables are inherently not visible outside the declaring library, so a leading underscore indicating private adds no value.

The following code produces this diagnostic because the parameter _s starts with an underscore:

Remove the underscore:

Don't put any logic in 'createState'.

The analyzer produces this diagnostic when an implementation of createState in a subclass of StatefulWidget contains any logic other than the return of the result of invoking a zero argument constructor.

The following code produces this diagnostic because the constructor invocation has arguments:

Rewrite the code so that createState doesn't contain any logic:

The referenced identifier is a wildcard.

The analyzer produces this diagnostic when either a parameter or local variable whose name consists of only underscores is referenced. Such names will become non-binding in a future version of the Dart language, making the reference illegal.

The following code produces this diagnostic because the name of the parameter consists of two underscores:

The following code produces this diagnostic because the name of the local variable consists of a single underscore:

If the variable or parameter is intended to be referenced, then give it a name that has at least one non-underscore character:

If the variable or parameter is not intended to be referenced, then replace the reference with a different expression:

The variable name '{0}' isn't a lowerCamelCase identifier.

The analyzer produces this diagnostic when the name of a class member, top-level declaration, variable, parameter, named parameter, or named constructor that isn't declared to be const, doesn't use the lowerCamelCase convention.

The following code produces this diagnostic because the top-level variable Count doesn't start with a lowercase letter:

Change the name in the declaration to follow the lowerCamelCase convention:

The null check operator shouldn't be used on a variable whose type is a potentially nullable type parameter.

The analyzer produces this diagnostic when a null check operator is used on a variable whose type is T?, where T is a type parameter that allows the type argument to be nullable (either has no bound or has a bound that is nullable).

Given a generic type parameter T which has a nullable bound, it is very easy to introduce erroneous null checks when working with a variable of type T?. Specifically, it is not uncommon to have T? x; and want to assert that x has been set to a valid value of type T. A common mistake is to do so using x!. This is almost always incorrect, because if T is a nullable type, x may validly hold null as a value of type T.

The following code produces this diagnostic because t has the type T? and T allows the type argument to be nullable (because it has no extends clause):

Use the type parameter to cast the variable:

Don't throw instances of classes that don't extend either 'Exception' or 'Error'.

The analyzer produces this diagnostic when the value being thrown isn't a subclass of either Exception or Error.

The following code produces this diagnostic because the string 'f' is being thrown:

Replace the value with an instance of a subclass of either Exception or Error:

Field overrides a field inherited from '{0}'.

The analyzer produces this diagnostic when a class defines a field that overrides a field from a superclass.

Overriding a field with another field causes the object to have two distinct fields, but because the fields have the same name only one of the fields can be referenced in a given scope. That can lead to confusion where a reference to one of the fields can be mistaken for a reference to the other.

The following code produces this diagnostic because the field f in B shadows the field f in A:

If the two fields are representing the same property, then remove the field from the subclass:

If the two fields should be distinct, then rename one of the fields:

If the two fields are related in some way, but can't be the same, then find a different way to implement the semantics you need.

The package name '{0}' isn't a lower_case_with_underscores identifier.

The analyzer produces this diagnostic when the name of a package doesn't use the lower_case_with_underscores naming convention.

The following code produces this diagnostic because the name of the package uses the lowerCamelCase naming convention:

Rewrite the name of the package using the lower_case_with_underscores naming convention:

The library name is not a dot-separated path prefixed by the package name.

The analyzer produces this diagnostic when a library has a name that doesn't follow these guidelines:

• Prefix all library names with the package name.
• Make the entry library have the same name as the package.
• For all other libraries in a package, after the package name add the dot-separated path to the library's Dart file.
• For libraries under lib, omit the top directory name.

For example, given a package named my_package, here are the library names for various files in the package:

Assuming that the file containing the following code is not in a file named special.dart in the lib directory of a package named something (which would be an exception to the rule), the analyzer produces this diagnostic because the name of the library doesn't conform to the guidelines above:

Change the name of the library to conform to the guidelines.

String literals shouldn't be concatenated by the '+' operator.

The analyzer produces this diagnostic when the + operator is used to concatenate two string literals.

The following code produces this diagnostic because two string literals are being concatenated by using the + operator:

Remove the operator:

Assert should be in the initializer list.

The analyzer produces this diagnostic when the body of a constructor begins with one or more assert statements.

The following code produces this diagnostic because the body of the constructor begins with an assert statement:

Move the assert to the initializer list, removing the body if there are only assert statements in it:

Missing a message in an assert.

The analyzer produces this diagnostic when an assert statement doesn't have a message.

The following code produces this diagnostic because there's no message in the assert statement:

Add a message to the assert statement:

Unnecessary constructor invocation.

The analyzer produces this diagnostic when a constructor is used to create a list, map, or set, but a literal would produce the same result.

The following code produces this diagnostic because the constructor for Map is being used to create a map that could also be created using a literal:

Use the literal representation:

The 'if' statement could be replaced by a null-aware assignment.

The analyzer produces this diagnostic when an assignment to a variable is conditional based on whether the variable has the value null and the ??= operator could be used instead.

The following code produces this diagnostic because the parameter s is being compared to null in order to determine whether to assign a different value:

Use the ??= operator instead of an explicit if statement:

Use 'const' with the constructor to improve performance.

The analyzer produces this diagnostic when an invocation of a const constructor isn't either preceded by const or in a constant context.

The following code produces this diagnostic because the invocation of the const constructor is neither prefixed by const nor in a constant context:

If the context can be made a constant context, then do so:

If the context can't be made a constant context, then add const before the constructor invocation:

Constructors in '@immutable' classes should be declared as 'const'.

The analyzer produces this diagnostic when a non-const constructor is found in a class that has the @immutable annotation.

The following code produces this diagnostic because the constructor in C isn't declared as const even though C has the @immutable annotation:

If the class really is intended to be immutable, then add the const modifier to the constructor:

If the class is mutable, then remove the @immutable annotation:

Use 'const' for final variables initialized to a constant value.

The analyzer produces this diagnostic when a top-level variable, static field, or local variable is marked as final and is initialized to a constant value.

The following code produces this diagnostic because the top-level variable v is both final and initialized to a constant value:

The following code produces this diagnostic because the static field f is both final and initialized to a constant value:

The following code produces this diagnostic because the local variable v is both final and initialized to a constant value:

Replace the keyword final with const and remove const from the initializer:

Use 'const' literals as arguments to constructors of '@immutable' classes.

The analyzer produces this diagnostic when a non-const list, map, or set literal is passed as an argument to a constructor declared in a class annotated with @immutable.

The following code produces this diagnostic because the list literal ([1]) is being passed to a constructor in an immutable class but isn't a constant list:

If the context can be made a constant context, then do so:

If the context can't be made a constant context but the constructor can be invoked using const, then add const before the constructor invocation:

If the context can't be made a constant context and the constructor can't be invoked using const, then add the keyword const before the collection literal:

Static method should be a constructor.

The analyzer produces this diagnostic when a static method returns a newly created instance of the class and could, therefore, be a constructor.

The following code produces this diagnostic because the static method all could be a constructor:

Convert the static method to a named constructor:

Always 'false' because 'indexOf' is always greater than or equal to -1.

Always 'true' because 'indexOf' is always greater than or equal to -1.

Unnecessary use of 'indexOf' to test for containment.

The analyzer produces this diagnostic when the method indexOf is used and the result is only compared with -1 or 0 in a way where the semantics are equivalent to using contains.

The following code produces this diagnostic because the condition in the if statement is checking to see whether the list contains the string:

Use contains instead, negating the condition when necessary:

Unnecessary use of single quotes.

The analyzer produces this diagnostic when a string literal uses single quotes (') when it could use double quotes (") without needing extra escapes and without hurting readability.

The following code produces this diagnostic because the string literal uses single quotes but doesn't need to:

Use double quotes in place of single quotes:

Unnecessary use of a block function body.

The analyzer produces this diagnostic when the body of a function consists of a single return statement with an expression.

The following code produces this diagnostic because the body of f has a single return statement:

If the body is complete, then replace the body with an expression body:

If the body isn't complete, then add the missing statements.

The private field {0} could be 'final'.

The analyzer produces this diagnostic when a private field is only assigned one time. The field can be initialized in multiple constructors and still be flagged because only one of those constructors can ever run.

The following code produces this diagnostic because the field _f is only assigned one time, in the field's initializer:

Mark the field final:

The pattern should be final.

The variable '{0}' should be final.

The analyzer produces this diagnostic when the loop variable in a for-each statement isn't marked as being final.

The following code produces this diagnostic because the loop variable e isn't marked as being final:

Add the modifier final to the loop variable, removing the var if there is one:

Local variables should be final.

The analyzer produces this diagnostic when a local variable isn't marked as being final.

The following code produces this diagnostic because the variable s isn't marked as being final:

Add the modifier final to the variable, removing the var if there is one:

The parameter '{0}' should be final.

The analyzer produces this diagnostic when a parameter of a constructor, method, function, or closure isn't marked as being final.

The following code produces this diagnostic because the parameter s isn't a final parameter:

Add the modifier final to the parameter:

Use 'for' elements when building maps from iterables.

The analyzer produces this diagnostic when Map.fromIterable is used to build a map that could be built using the for element.

The following code produces this diagnostic because fromIterable is being used to build a map that could be built using a for element:

Use a for element to build the map:

Use 'forEach' rather than a 'for' loop to apply a function to every element.

The analyzer produces this diagnostic when a for loop is used to operate on every member of a collection and the method forEach could be used instead.

The following code produces this diagnostic because a for loop is being used to invoke a single function for each key in m:

Replace the for loop with an invocation of forEach:

Use a function declaration rather than a variable assignment to bind a function to a name.

The analyzer produces this diagnostic when a closure is assigned to a local variable and the local variable is not re-assigned anywhere.

The following code produces this diagnostic because the local variable f is initialized to be a closure and isn't assigned any other value:

Replace the local variable with a local function:

Use the generic function type syntax in 'typedef's.

The analyzer produces this diagnostic when a typedef is written using the older syntax for function type aliases in which the name being declared is embedded in the function type.

The following code produces this diagnostic because it uses the older syntax:

Rewrite the typedef to use the newer syntax:

Use the '??' operator rather than '?:' when testing for 'null'.

The analyzer produces this diagnostic when a conditional expression (using the ?: operator) is used to select a different value when a local variable is null.

The following code produces this diagnostic because the variable s is being compared to null so that a different value can be returned when s is null:

Use the if-null operator instead:

Use an initializing formal to assign a parameter to a field.

The analyzer produces this diagnostic when a constructor parameter is used to initialize a field without modification.

The following code produces this diagnostic because the parameter c is only used to set the field c:

Use an initializing formal parameter to initialize the field:

The addition of a list item could be inlined.

The addition of multiple list items could be inlined.

The analyzer produces this diagnostic when the methods add and addAll are invoked on a list literal where the elements being added could be included in the list literal.

The following code produces this diagnostic because the add method is being used to add b, when it could have been included directly in the list literal:

The following code produces this diagnostic because the addAll method is being used to add the elements of b, when it could have been included directly in the list literal:

If the add method is being used, then make the argument an element of the list and remove the invocation:

If the addAll method is being used, then use the spread operator on the argument to add its elements to the list and remove the invocation:

Use interpolation to compose strings and values.

The analyzer produces this diagnostic when string literals and computed strings are being concatenated using the + operator, but string interpolation would achieve the same result.

The following code produces this diagnostic because the String s is concatenated with other strings using the + operator:

Use string interpolation:

The comparison is always 'false' because the length is always greater than or equal to 0.

The comparison is always 'true' because the length is always greater than or equal to 0.

Use 'isEmpty' instead of 'length' to test whether the collection is empty.

Use 'isNotEmpty' instead of 'length' to test whether the collection is empty.

The analyzer produces this diagnostic when the result of invoking either Iterable.length or Map.length is compared for equality with zero (0).

The following code produces this diagnostic because the result of invoking length is checked for equality with zero:

Replace the use of length with a use of either isEmpty or isNotEmpty:

Use 'isNotEmpty' rather than negating the result of 'isEmpty'.

The analyzer produces this diagnostic when the result of invoking Iterable.isEmpty or Map.isEmpty is negated.

The following code produces this diagnostic because the result of invoking Iterable.isEmpty is negated:

Rewrite the code to use isNotEmpty:

Use the 'is!' operator rather than negating the value of the 'is' operator.

The analyzer produces this diagnostic when the prefix ! operator is used to negate the result of an is test.

The following code produces this diagnostic because the result of testing to see whether o is a String is negated using the prefix ! operator:

Use the is! operator instead:

Use 'whereType' to select elements of a given type.

The analyzer produces this diagnostic when the method Iterable.where is being used to filter elements based on their type.

The following code produces this diagnostic because the method where is being used to access only the strings within the iterable:

Rewrite the code to use whereType:

This might also allow you to tighten the types in your code or remove other type checks.

Use the null-aware operator '?.' rather than an explicit 'null' comparison.

The analyzer produces this diagnostic when a comparison with null is used to guard a member reference, and null is used as a result when the guarded target is null.

The following code produces this diagnostic because the invocation of length is guarded by a null comparison even though the default value is null:

Use a null-aware access operator instead:

Use relative imports for files in the 'lib' directory.

The analyzer produces this diagnostic when an import in a library inside the lib directory uses a package: URI to refer to another library in the same package.

The following code produces this diagnostic because it uses a package: URI when a relative URI could have been used:

Use a relative URI to import the library:

Unnecessary use of double quotes.

The analyzer produces this diagnostic when a string literal uses double quotes (") when it could use single quotes (') without needing extra escapes and without hurting readability.

The following code produces this diagnostic because the string literal uses double quotes but doesn't need to:

Use single quotes in place of double quotes:

An uninitialized field should have an explicit type annotation.

An uninitialized variable should have an explicit type annotation.

The analyzer produces this diagnostic when a variable without an initializer doesn't have an explicit type annotation.

Without either a type annotation or an initializer, a variable has the type dynamic, which allows any value to be assigned to the variable, often causing hard to identify bugs.

The following code produces this diagnostic because the variable r doesn't have either a type annotation or an initializer:

If the variable can be initialized, then add an initializer:

If the variable can't be initialized, then add an explicit type annotation:

Unnecessary use of the type 'Null'.

The analyzer produces this diagnostic when Null is used in a location where void would be a valid choice.

The following code produces this diagnostic because the function f is declared to return null (at some future time):

Replace the use of Null with a use of void:

Missing a deprecation message.

The analyzer produces this diagnostic when a deprecated annotation is used instead of the Deprecated annotation.

The following code produces this diagnostic because the function f is annotated with deprecated:

Convert the code to use the longer form:

Missing documentation for a public member.

The analyzer produces this diagnostic when the declaration of part of the public API of a package doesn't have a documentation comment.

The following code produces this diagnostic because the class C doesn't have a documentation comment:

Add a documentation comment.

The getter '{0}' recursively returns itself.

The analyzer produces this diagnostic when a getter invokes itself, resulting in an infinite loop.

The following code produces this diagnostic because the getter count invokes itself:

Change the getter to not invoke itself:

The '{0}' protocol shouldn't be used because it isn't secure.

The analyzer produces this diagnostic when a URL in a pubspec.yaml file is using a non-secure scheme, such as http.

The following code produces this diagnostic because the pubspec.yaml file contains an http URL:

Change the scheme of the URL to use a secure scheme, such as https:

Use a 'SizedBox' to add whitespace to a layout.

The analyzer produces this diagnostic when a Container is created using only the height and/or width arguments.

The following code produces this diagnostic because the Container has only the width argument:

Replace the Container with a SizedBox of the same dimensions:

Use 'SizedBox.{0}' to avoid needing to specify the 'height' and 'width'.

The analyzer produces this diagnostic when a SizedBox constructor invocation specifies the values of both height and width as either 0.0 or double.infinity.

The following code produces this diagnostic because both the height and width are 0.0:

The following code produces this diagnostic because both the height and width are double.infinity:

If both are 0.0, then use SizedBox.shrink:

If both are double.infinity, then use SizedBox.expand:

Use the end-of-line form ('///') for doc comments.

The analyzer produces this diagnostic when a documentation comment uses the block comment style (delimited by /** and */).

The following code produces this diagnostic because the documentation comment for f uses a block comment style:

Use an end-of-line comment style:

The '{0}' argument should be last in widget constructor invocations.

The analyzer produces this diagnostic when the child or children argument isn't the last argument in an invocation of a widget class' constructor. An exception is made if all of the arguments after the child or children argument are function expressions.

The following code produces this diagnostic because the child argument isn't the last argument in the invocation of the Center constructor:

Move the child or children argument to be last:

Constructor declarations should be before non-constructor declarations.

The analyzer produces this diagnostic when a constructor declaration is preceded by one or more non-constructor declarations.

The following code produces this diagnostic because the constructor for C appears after the method m:

Move all of the constructor declarations before any other declarations:

Dependencies not sorted alphabetically.

The analyzer produces this diagnostic when the keys in a dependency map in the pubspec.yaml file aren't sorted alphabetically. The dependency maps that are checked are the dependencies, dev_dependencies, and dependency_overrides maps.

The following code produces this diagnostic because the entries in the dependencies map are not sorted:

Sort the entries:

Invalid location for the unnamed constructor.

The analyzer produces this diagnostic when an unnamed constructor appears after a named constructor.

The following code produces this diagnostic because the unnamed constructor is after the named constructor:

Move the unnamed constructor before any other constructors:

Missing type test for '{0}' in '=='.

The analyzer produces this diagnostic when an override of the == operator doesn't include a type test on the value of the parameter.

The following code produces this diagnostic because other is not type tested:

Perform an is test as part of computing the return value:

Use of '{0}' in 'finally' block.

The analyzer produces this diagnostic when a throw statement is found inside a finally block.

The following code produces this diagnostic because there is a throw statement inside a finally block:

Rewrite the code so that the throw statement isn't inside a finally block:

Use a type annotation rather than 'assert' to enforce non-nullability.

The analyzer produces this diagnostic when an assert is being used in the initializer list of a constructor to ensure that only a non-null value is being used to initialize a field.

The following code produces this diagnostic because an assert is being used to catch an error that could be caught by the type system:

Remove the assert and add the non-nullable type before the initializing formal:

Missing type annotation on a public API.

The analyzer produces this diagnostic when the declaration of part of the public API of a package doesn't have explicit type annotations.

The following code produces this diagnostic because the function f doesn't have an explicit return type and the parameters x and y don't have explicit types:

Add type annotations to the API:

Don't needlessly type annotate initializing formals.

The analyzer produces this diagnostic when an initializing formal parameter (this.x) or a super parameter (super.x) has an explicit type annotation that is the same as the field or overridden parameter.

If a constructor parameter is using this.x to initialize a field, then the type of the parameter is implicitly the same type as the field. If a constructor parameter is using super.x to forward to a super constructor, then the type of the parameter is implicitly the same as the super constructor parameter.

The following code produces this diagnostic because the parameter this.c has an explicit type that is the same as the field c:

The following code produces this diagnostic because the parameter super.a has an explicit type that is the same as the parameter a from the superclass:

Remove the type annotation from the parameter:

Use 'TypeName _' instead of a type literal.

The analyzer produces this diagnostic when a type literal appears as a pattern.

The following code produces this diagnostic because a type literal is used as a constant pattern:

If the type literal is intended to match an object of the given type, then use either a variable pattern:

Or an object pattern:

If the type literal is intended to match the type literal, then write it as a constant pattern:

Missing an 'await' for the 'Future' computed by this expression.

The analyzer produces this diagnostic on an expression with a Future type, only in a few specific cases:

• when the expression is itself a statement (like f();),
• when the expression is part of a cascade (like C()..f()),
• when the expression is a String interpolation (like '${f()}').

The analyzer only produces this diagnostic on expressions inside an async or async* function.

The two common corrections are to 'await' the expression, or to wrap the expression in a call to unawaited().

The following code produces this diagnostic because the function g returns a future, but the future isn't awaited:

If the future needs to complete before the following code is executed, then add an await before the invocation:

If the future doesn't need to complete before the following code is executed, then wrap the Future-returning invocation in an invocation of the unawaited function:

Angle brackets will be interpreted as HTML.

The analyzer produces this diagnostic when a documentation comment contains angle bracketed text (<...>) that isn't one of the allowed exceptions.

Such text is interpreted by markdown to be an HTML tag, which is rarely what was intended.

See the lint rule description for the list of allowed exceptions.

The following code produces this diagnostic because the documentation comment contains the text <int>, which isn't one of the allowed exceptions:

If the text was intended to be part of a code span, then add backticks around the code:

If the text was intended to be part of a link, then add square brackets around the code:

If the text was intended to be printed as-is, including the angle brackets, then add backslash escapes before the angle brackets:

Unnecessary braces in a string interpolation.

The analyzer produces this diagnostic when a string interpolation with braces is used to interpolate a simple identifier and isn't followed by alphanumeric text.

The following code produces this diagnostic because the interpolation element ${s} uses braces when they are not necessary:

Remove the unnecessary braces:

Unnecessary 'const' keyword.

The analyzer produces this diagnostic when the keyword const is used in a constant context. The keyword isn't required because it's implied.

The following code produces this diagnostic because the keyword const in the list literal isn't needed:

The list is implicitly const because of the keyword const on the variable declaration.

Remove the unnecessary keyword:

Unnecessary '.new' constructor name.

The analyzer produces this diagnostic when a reference to an unnamed constructor uses .new. The only place where .new is required is in a constructor tear-off.

The following code produces this diagnostic because .new is being used to refer to the unnamed constructor where it isn't required:

Remove the unnecessary .new:

Local variables should not be marked as 'final'.

The analyzer produces this diagnostic when a local variable is marked as being final.

The following code produces this diagnostic because the local variable c is marked as being final:

If the variable doesn't have a type annotation, then replace the final with var:

If the variable has a type annotation, then remove the final modifier:

Unnecessary use of getter and setter to wrap a field.

The analyzer produces this diagnostic when a getter and setter pair returns and sets the value of a field without any additional processing.

The following code produces this diagnostic because the getter/setter pair named c only expose the field named _c:

Make the field public and remove the getter and setter:

The diagnostic '{0}' isn't produced at this location so it doesn't need to be ignored.

The diagnostic '{0}' isn't produced in this file so it doesn't need to be ignored.

The analyzer produces this diagnostic when an ignore is specified to ignore a diagnostic that isn't produced.

The following code produces this diagnostic because the unused_local_variable diagnostic isn't reported at the ignored location:

Remove the ignore comment:

Closure should be a tearoff.

The analyzer produces this diagnostic when a closure (lambda) could be replaced by a tear-off.

The following code produces this diagnostic because the closure passed to forEach contains only an invocation of the function print with the parameter of the closure:

Replace the closure with a tear-off of the function or method being invoked with the closure:

Unnecessary 'late' modifier.

The analyzer produces this diagnostic when a top-level variable or static field with an initializer is marked as late. Top-level variables and static fields are implicitly late, so they don't need to be explicitly marked.

The following code produces this diagnostic because the static field c has the modifier late even though it has an initializer:

Remove the keyword late:

Library names are not necessary.

The analyzer produces this diagnostic when a library directive specifies a name.

The following code produces this diagnostic because the library directive includes a name:

Remove the name from the library directive:

If the library has any parts, then any part of declarations that use the library name should be updated to use the URI of the library instead.

Unnecessary 'new' keyword.

The analyzer produces this diagnostic when the keyword new is used to invoke a constructor.

The following code produces this diagnostic because the keyword new is used to invoke the unnamed constructor from Object:

Remove the keyword new:

Unnecessary assignment of 'null'.

The analyzer produces this diagnostic when the right-hand side of a null-aware assignment is the null literal.

The following code produces this diagnostic because the null aware operator is being used to assign null to s when s is already null:

If a non-null value should be assigned to the left-hand operand, then change the right-hand side:

If there is no non-null value to assign to the left-hand operand, then remove the assignment:

Unnecessary use of a null-aware operator to invoke an extension method on a nullable type.

The analyzer produces this diagnostic when a null-aware operator is used to invoke an extension method on an extension whose type is nullable.

The following code produces this diagnostic because the extension method m is invoked using ?. when it doesn't need to be:

If it isn't a requirement not invoke the method when the receiver is null, then remove the question mark from the invocation:

Unnecessary use of a null check ('!').

The analyzer produces this diagnostic when a null check operator (!) is used in a context where a nullable value is acceptable.

The following code produces this diagnostic because a null check is being used even though null is a valid value to return:

Remove the null check operator:

Unnecessary use of '??' with 'null'.

The analyzer produces this diagnostic when the right operand of the ?? operator is the literal null.

The following code produces this diagnostic because the right-hand operand of the ?? operator is null:

If a non-null value should be used for the right-hand operand, then change the right-hand side:

If there is no non-null value to use for the right-hand operand, then remove the operator and the right-hand operand:

Type could be non-nullable.

The analyzer produces this diagnostic when a final field or variable has a nullable type but is initialized to a non-nullable value.

The following code produces this diagnostic because the final variable i has a nullable type (int?), but can never be null:

Make the type non-nullable:

Unnecessary override.

The analyzer produces this diagnostic when an instance member overrides an inherited member but only invokes the overridden member with exactly the same arguments.

The following code produces this diagnostic because the method D.m doesn't do anything other than invoke the overridden method:

If the method should do something more than what the overridden method does, then implement the missing functionality:

If the overridden method should be modified by changing the return type or one or more of the parameter types, making one of the parameters covariant, having a documentation comment, or by having additional annotations, then update the code:

If the overriding method doesn't change or enhance the semantics of the code, then remove it:

Unnecessary use of parentheses.

The analyzer produces this diagnostic when parentheses are used where they do not affect the semantics of the code.

The following code produces this diagnostic because the parentheses around the binary expression are not necessary:

Remove the unnecessary parentheses:

Unnecessary use of a raw string.

The analyzer produces this diagnostic when a string literal is marked as being raw (is prefixed with an r), but making the string raw doesn't change the value of the string.

The following code produces this diagnostic because the string literal will have the same value without the r as it does with the r:

Remove the r in front of the string literal:

Unnecessary statement.

The analyzer produces this diagnostic when an expression statement has no clear effect.

The following code produces this diagnostic because the addition of the returned values from the two invocations has no clear effect:

If the expression doesn't need to be computed, then remove it:

If the value of the expression is needed, then make use of it, possibly assigning it to a local variable first:

If portions of the expression need to be executed, then remove the unnecessary portions:

Unnecessary escape in string literal.

The analyzer produces this diagnostic when characters in a string are escaped when escaping them is unnecessary.

The following code produces this diagnostic because single quotes don't need to be escaped inside strings delimited by double quotes:

Remove the unnecessary backslashes:

Unnecessary use of string interpolation.

The analyzer produces this diagnostic when a string literal contains a single interpolation of a String-valued variable and no other characters.

The following code produces this diagnostic because the string literal contains a single interpolation and doesn't contain any character outside the interpolation:

Replace the string literal with the content of the interpolation:

Unnecessary 'this.' qualifier.

The analyzer produces this diagnostic when the keyword this is used to access a member that isn't shadowed.

The following code produces this diagnostic because the use of this to access the field _f isn't necessary:

Remove the this.:

Unnecessary use of 'toList' in a spread.

The analyzer produces this diagnostic when toList is used to convert an Iterable to a List just before a spread operator is applied to the list. The spread operator can be applied to any Iterable, so the conversion isn't necessary.

The following code produces this diagnostic because toList is invoked on the result of map, which is an Iterable that the spread operator could be applied to directly:

Remove the invocation of toList:

Unnecessary use of multiple underscores.

The analyzer produces this diagnostic when an unused variable is named with multiple underscores (for example __). A single _ wildcard variable can be used instead.

The following code produces this diagnostic because the __ parameter is unused:

Replace the name with a single underscore:

The type of the operand ('{0}') isn't a subtype or a supertype of the value being matched ('{1}').

The type of the right operand ('{0}') isn't a subtype or a supertype of the left operand ('{1}').

The analyzer produces this diagnostic when two objects are being compared and neither of the static types of the two objects is a subtype of the other.

Such a comparison will usually return false and might not reflect the programmer's intent.

There can be false positives. For example, a class named Point might have subclasses named CartesianPoint and PolarPoint, neither of which is a subtype of the other, but it might still be appropriate to test the equality of instances.

As a concrete case, the classes Int64 and Int32 from package:fixnum allow comparing instances to an int provided the int is on the right-hand side. This case is specifically allowed by the diagnostic, but other such cases are not.

The following code produces this diagnostic because the string s is being compared to the integer 1:

Replace one of the operands with something compatible with the other operand:

This type is unsafe: a type parameter occurs in a non-covariant position.

The analyzer produces this diagnostic when an instance member has a result type which is contravariant or invariant in a type parameter of the enclosing declaration. The result type of a variable is its type, and the result type of a getter or method is its return type. This lint warns against such members because they are likely to cause a failing type check at run time, with no static warning or error at the call site.

The following code produces this diagnostic because X occurs as a parameter type in the type of f, which is a contravariant occurrence of this type parameter:

This is unsafe: If c has static type C<num> and run-time type C<int> then c.f will throw. Hence, every invocation c.f(a) will also throw, even in the case where a has a correct type as an argument to c.f.

If the linted member is or can be private then you may be able to enforce that it is never accessed on any other receiver than this. This is sufficient to ensure that that the run-time type error does not occur. For example:

You can eliminate the unsafe variance by using a more general type for the linted member. In this case you may need to check the run-time type and perform a downcast at call sites.

If c has static type C<num> then you may test the type. For example, c.f is bool Function(num). You may safely call it with an argument of type num if it has that type.

You can also eliminate the unsafe variance by using a much more general type like Function, which is essentially the type dynamic for functions.

This will make c.f(a) dynamically safe: It will throw if and only if the argument a does not have the type required by the function. This is better than the original version because it will not throw because of a mismatched static type. It only throws when it must throw for soundness reasons.

Don't use 'BuildContext's across async gaps, guarded by an unrelated 'mounted' check.

Don't use 'BuildContext's across async gaps.

The analyzer produces this diagnostic when a BuildContext is referenced by a StatefulWidget after an asynchronous gap without first checking the mounted property.

Storing a BuildContext for later use can lead to difficult-to-diagnose crashes. Asynchronous gaps implicitly store a BuildContext, making them easy to overlook for diagnosis.

The following code produces this diagnostic because the context is passed to a constructor after the await:

If you can remove the asynchronous gap, do so:

If you can't remove the asynchronous gap, then use mounted to guard the use of the context:

Use a 'ColoredBox' rather than a 'Container' with only a 'Color'.

The analyzer produces this diagnostic when a Container is created that only sets the color.

The following code produces this diagnostic because the only attribute of the container that is set is the color:

Replace the Container with a ColoredBox:

Use 'DecoratedBox' rather than a 'Container' with only a 'Decoration'.

The analyzer produces this diagnostic when a Container is created that only sets the decoration.

The following code produces this diagnostic because the only attribute of the container that is set is the decoration:

Replace the Container with a DecoratedBox:

Instances of 'Color' should be created using an 8-digit hexadecimal integer (such as '0xFFFFFFFF').

The analyzer produces this diagnostic when the argument to the constructor of the Color class is a literal integer that isn't represented as an 8-digit hexadecimal integer.

The following code produces this diagnostic because the argument (1) isn't represented as an 8-digit hexadecimal integer:

Convert the representation to be an 8-digit hexadecimal integer:

Use the generic function type syntax to declare the parameter '{0}'.

The analyzer produces this diagnostic when the older style function-valued parameter syntax is used.

The following code produces this diagnostic because the function-valued parameter f is declared using an older style syntax:

Use the generic function type syntax to declare the parameter:

Use an if-null operator to convert a 'null' to a 'bool'.

The analyzer produces this diagnostic when a nullable bool-valued expression is compared (using == or !=) to a boolean literal.

The following code produces this diagnostic because the nullable boolean variable b is compared to true:

Rewrite the condition to use ?? instead:

Constructors for public widgets should have a named 'key' parameter.

The analyzer produces this diagnostic when a constructor in a subclass of Widget that isn't private to its library doesn't have a parameter named key.

The following code produces this diagnostic because the constructor for the class MyWidget doesn't have a parameter named key:

The following code produces this diagnostic because the default constructor for the class MyWidget doesn't have a parameter named key:

Add a parameter named key to the constructor, explicitly declaring the constructor if necessary:

Use 'late' for private members with a non-nullable type.

The analyzer produces this diagnostic when a private field or variable is marked as being nullable, but every reference assumes that the variable is never null.

The following code produces this diagnostic because the private top-level variable _i is nullable, but every reference assumes that it will not be null:

Mark the variable or field as being both non-nullable and late to indicate that it will always be assigned a non-null:

Use the constant '{0}' rather than a constructor returning the same object.

The analyzer produces this diagnostic when a constant is created with the same value as a known const variable.

The following code produces this diagnostic because there is a known const field (Duration.zero) whose value is the same as what the constructor invocation will evaluate to:

Replace the constructor invocation with a reference to the known const variable:

Use the null-aware marker '?' rather than a null check via an 'if'.

The analyzer produces this diagnostic when a null check is used instead of a null-aware marker inside of a collection literal.

The following code produces this diagnostic because a null check is used to decide whether x should be inserted into the list, while the null-aware marker '?' would be less brittle and less verbose.

Replace the null-check with the null-aware marker '?':

Use a raw string to avoid using escapes.

The analyzer produces this diagnostic when a string literal containing escapes, and no interpolations, could be marked as being raw in order to avoid the need for the escapes.

The following code produces this diagnostic because the string contains escaped characters that wouldn't need to be escaped if the string is made a raw string:

Mark the string as being raw and remove the unnecessary backslashes:

Use 'rethrow' to rethrow a caught exception.

The analyzer produces this diagnostic when a caught exception is thrown using a throw expression rather than a rethrow statement.

The following code produces this diagnostic because the caught exception e is thrown using a throw expression:

Use rethrow instead of throw:

The method is used to change a property.

The analyzer produces this diagnostic when a method is used to set the value of a field, or a function is used to set the value of a top-level variable, and nothing else.

The following code produces this diagnostic because the method setF is used to set the value of the field _f and does no other work:

Convert the method to a setter:

Use a string buffer rather than '+' to compose strings.

The analyzer produces this diagnostic when values are concatenated to a string inside a loop without using a StringBuffer to do the concatenation.

The following code produces this diagnostic because the string result is computed by repeated concatenation within the for loop:

Use a StringBuffer to compute the result:

The part-of directive uses a library name.

The analyzer produces this diagnostic when a part of directive uses a library name to refer to the library that the part is a part of.

Given a file named lib.dart that contains the following:

The following code produces this diagnostic because the part of directive uses the name of the library rather than the URI of the library it's part of:

Use a URI to reference the library:

Parameter '{0}' could be a super parameter.

Parameters '{0}' could be super parameters.

The analyzer produces this diagnostic when a parameter to a constructor is passed to a super constructor without being referenced or modified and a super parameter isn't used.

The following code produces this diagnostic because the parameters of the constructor for B are only used as arguments to the super constructor:

Use a super parameter to pass the arguments:

Use truncating division.

The analyzer produces this diagnostic when the result of dividing two numbers is converted to an integer using toInt.

Dart has a built-in integer division operator that is both more efficient and more concise.

The following code produces this diagnostic because the result of dividing x and y is converted to an integer using toInt:

Use the integer division operator (~/):

Invalid regular expression syntax.

The analyzer produces this diagnostic when the string passed to the default constructor of the class RegExp doesn't contain a valid regular expression.

A regular expression created with invalid syntax will throw a FormatException at runtime.

The following code produces this diagnostic because the regular expression isn't valid:

Fix the regular expression:

Assignment to a variable of type 'void'.

The analyzer produces this diagnostic when a value is assigned to a variable of type void.

It isn't possible to access the value of such a variable, so the assignment has no value.

The following code produces this diagnostic because the field value has the type void, but a value is being assigned to it:

The following code produces this diagnostic because the type of the parameter p in the method m is void, but a value is being assigned to it in the invocation:

If the type of the variable is incorrect, then change the type of the variable:

If the type of the variable is correct, then remove the assignment:

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