diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t13.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t13.dart
new file mode 100644
index 0000000..dba767a
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t13.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  final int values = 42;
+//          ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  final List<E2> values = [];
+//               ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t14.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t14.dart
new file mode 100644
index 0000000..2507357
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t14.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  int values() => 42;
+//    ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  List<E2> values() => [];
+//         ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t15.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t15.dart
new file mode 100644
index 0000000..9f231e8
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t15.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  int get values => 42;
+//        ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  List<E2> get values => [];
+//             ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t16.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t16.dart
new file mode 100644
index 0000000..69a0456
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t16.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  void set values(int val) {}
+//         ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  void set values(List<E2> val) {}
+//         ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t01.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t01.dart
new file mode 100644
index 0000000..6899e0d
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t01.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  final int index = 42;
+//          ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  final List<E2> index = [];
+//               ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t02.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t02.dart
new file mode 100644
index 0000000..b06b500
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t02.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  int index() => 42;
+//    ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  List<E2> index() => [];
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t03.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t03.dart
new file mode 100644
index 0000000..e3a527e
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t03.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  int get index => 42;
+//        ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  List<E2> get index => [];
+//             ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t04.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t04.dart
new file mode 100644
index 0000000..17d1ca0
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t04.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a class declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+abstract class E1 extends Enum {
+  void set index(int val) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+abstract class E2 extends Enum {
+  void set index(List<E2> val) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t05.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t05.dart
new file mode 100644
index 0000000..dee1605
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t05.dart
@@ -0,0 +1,80 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a enum declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+enum E1 {
+  e1,
+  e2;
+
+  final int index = 42;
+//          ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+enum E2 {
+  e1(42),
+  e2(0);
+
+  const E2(int i);
+  final List<E2> index = [];
+//               ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t06.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t06.dart
new file mode 100644
index 0000000..dd0cb24
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t06.dart
@@ -0,0 +1,80 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a enum declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+enum E1 {
+  e1,
+  e2;
+
+  int index() => 42;
+//    ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+enum E2 {
+  e1(42),
+  e2(0);
+
+  const E2(int i);
+  List<E2> index() => [];
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t07.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t07.dart
new file mode 100644
index 0000000..1bfc803
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t07.dart
@@ -0,0 +1,80 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a enum declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+enum E1 {
+  e1,
+  e2;
+
+  int get index => 42;
+//        ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+enum E2 {
+  e1(42),
+  e2(0);
+
+  const E2(int i);
+  List<E2> get index => [];
+//             ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t08.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t08.dart
new file mode 100644
index 0000000..8520314
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t08.dart
@@ -0,0 +1,80 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @description Check that it's a compile-time error if a enum declaration has
+/// Enum as a superinterface, and it declares a non-abstract instance member
+/// named `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+enum E1 {
+  e1,
+  e2;
+
+  void set index(int val) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+enum E2 {
+  e1(42),
+  e2(0);
+
+  const E2(int i);
+  void set index(List<E2> val) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t09.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t09.dart
new file mode 100644
index 0000000..b7e15e7
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t09.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @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 `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+mixin M1 on Enum {
+  final int index = 42;
+//          ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+mixin M2 on Enum {
+  final List<String> index = [];
+//                   ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(M1);
+  print(M2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t10.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t10.dart
new file mode 100644
index 0000000..eb144ad
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t10.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @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 `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+mixin M1 on Enum {
+  int index() => 42;
+//    ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+mixin M2 on Enum {
+  List<String> index() => [];
+//             ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(M1);
+  print(M2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t11.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t11.dart
new file mode 100644
index 0000000..2fd17de
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t11.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @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 `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+mixin M1 on Enum {
+  int get index => 42;
+//        ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+mixin M2 on Enum {
+  List<String> get index => [];
+//                 ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(M1);
+  print(M2);
+}
diff --git a/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t12.dart b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t12.dart
new file mode 100644
index 0000000..aef5512
--- /dev/null
+++ b/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t12.dart
@@ -0,0 +1,73 @@
+// 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 non-abstract class has Enum as a
+/// superinterface (directly or transitively) unless it is the corresponding
+/// class of an enum declaration.
+///
+/// It is a compile-time error if a class implements, extends or mixes-in the
+/// class or interface introduced by an enum declaration. (An enum class can’t
+/// be used as a mixin since it is not a mixin declaration and the class has a
+/// superclass other than Object, but we include “mixes-in” for completeness.)
+///
+/// It's a compile-time error if a class or mixin declaration has Enum as a
+/// superinterface and the interface of the declarations contains an instance
+/// member with the name values, whether declared or inherited. If any concrete
+/// class implements this interface, it will be an enum declaration class, and
+/// then the values member would conflict with the static values constant getter
+/// that is automatically added to enum declaration classes. Such an instance
+/// values declaration is either useless or wrong, so we disallow it entirely.
+///
+/// It's a compile-time error if a class, mixin or enum declaration has Enum as
+/// a superinterface, and it declares a non-abstract instance member named index.
+/// That member would override the index getter inherited from Enum, and we
+/// currently do not allow that.
+///
+/// Those restrictions allows abstract classes (interfaces) which implements
+/// Enum in order to have the int index; getter member available, and it allows
+/// mixin declarations to use Enum as an on type because mixin declarations
+/// cannot be instantiated directly.
+///
+/// The restrictions still ensure enum values are the only object instances
+/// which implements Enum, while making it valid to declare abstract class
+/// MyInterface implements Enum and mixin MyMixin on Enum for interfaces and
+/// mixins intended to be used in declaring enum classes. It's also impossible
+/// to override or prevent the instance index and static values members without
+/// causing a compile-time error. Say implementing an interface with
+/// `Never get index;` as a member, then because it's not possible to override
+/// `int get index;` from Enum, the resulting class does not implement its
+/// interface and is a compile-time error.
+///
+/// @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 `index`.
+/// @author sgrekhov@unipro.ru
+
+// SharedOptions=--enable-experiment=enhanced-enums
+
+mixin M1 on Enum {
+  void set index(int v) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+mixin M2 on Enum {
+  void set index(List<String> val) {}
+//         ^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(M1);
+  print(M2);
+}