deferred_import_of_extension

Deferred library imports must hide all extension declarations.

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:

class C {}
​
extension E on String {
  int get size => length;
}

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

import 'a.dart' deferred as a;
​
void f() {
  a.C();
}

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:

import 'a.dart' deferred as a show C;
​
void f() {
  a.C();
}

Adding a hide clause would look like this:

import 'a.dart' deferred as a hide E;
​
void f() {
  a.C();
}

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:

import 'a.dart' as a;
​
void f() {
  a.C();
}