invalid_reopen_annotation

The annotation '@reopen' can only be applied to a class that opens capabilities that the supertype intentionally disallows.

The analyzer produces this diagnostic when a @reopen annotation has been placed on a class or mixin that does not remove restrictions placed on the superclass.

The following code produces this diagnostic because the class B is annotated with @reopen even though it doesn't expand the ability of A to be subclassed:

import 'package:meta/meta.dart';

sealed class A {}

@reopen
class B extends A {}

If the superclass should be restricted in a way that the subclass would change, then modify the superclass to reflect those restrictions:

import 'package:meta/meta.dart';

interface class A {}

@reopen
class B extends A {}

If the superclass is correct, then remove the annotation:

sealed class A {}

class B extends A {}