LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t27.dart

// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// @assertion It’s currently a compile-time error for a class to implement,
/// extend or mix-in the Enum class.
///
/// Because we want to allow interfaces and mixins that are intended to be
/// applied to enum declarations, and therefore to assume Enum to be a
/// superclass, we loosen that restriction to:
/// ...
/// It's a compile-time error if a class or mixin declaration has Enum as a
/// superinterface, and that class or mixin declares or inherits a concrete
/// instance member named `index`, `hashCode` or `==` (an `operator ==`
/// declaration). That `index` member could override the `index` getter
/// inherited from `Enum`, and we currently do not allow that. The `hashCode`
/// and `operator ==` declarations would prevent the enum class from having
/// "primitive equality", and we want to ensure that enums can be used in
/// switches.
///
/// @description Check that it's a compile-time error if a mixin declaration has
/// Enum as a superinterface, and it declares a non-abstract instance member
/// named `operator ==`.
/// @author sgrekhov@unipro.ru

// SharedOptions=--enable-experiment=enhanced-enums

mixin M1 on Enum {
  bool operator ==(Object other) {
//     ^^^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    return true;
  }
}

mixin M2 on Enum {
  bool operator ==(covariant E2 other) {
//    ^^^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    return false;
  }
}

main() {
  print(M1);
  print(M2);
}