mixin_inherits_from_not_object

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:

//@dart=2.19
class A {}
​
class B extends A {}
​
class C with B {}

If the class being used as a mixin can be changed to extend Object, then change it:

//@dart=2.19
class A {}
​
class B {}
​
class C with B {}

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:

class A {}
​
class B extends A {}
​
class C extends B {}

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:

class A {}
​
mixin M on A {}
​
class B extends A with M {}
​
class C extends A with M {}