mixin_on_sealed_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:

import 'package:meta/meta.dart';

@sealed
class C {}

Then, the following code, when in a package other than p, produces this diagnostic:

import 'package:p/p.dart';

mixin M on C {}

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.

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.