Add tests for the transitivity of the base modifier when applied to

base mixins and base mixin classes.

Change-Id: Idfe7576f38382c5e9a0ff968aae00f48990e1d38
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290351
Commit-Queue: Leaf Petersen <leafp@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>

tests/language/class_modifiers/base_transitivity/base_class_different_library_test.dart

@@ -41,11 +41,19 @@ sealed class SealedExtendWith extends BaseClass with _MixinOnObject {}
 
 // Extending via an anonymous mixin application class.
 final class FinalExtendApplication = BaseClass with _MixinOnObject;
+
 base class BaseExtendApplication = BaseClass with _MixinOnObject;
+
 sealed class SealedExtendApplication = BaseClass with _MixinOnObject;
 
 /// BaseClass can be an `on` type, so long as the subtype is base.
 
 base mixin BaseMixinOn on BaseClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/base_class_same_library_test.dart

@@ -119,4 +119,10 @@ sealed class SealedMixinImplementApplied extends Object
 
 base mixin BaseMixinOn on BaseClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/base_mixin_class_different_library_error_test.dart

@@ -0,0 +1,426 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the invalid uses of a base mixin class defined in a different library.
+
+import "shared_library_definitions.dart" show SimpleClass, BaseMixinClass;
+
+mixin _MixinOnObject {}
+
+/// It is an error if BaseMixinClass is extended by something which is not base,
+/// final or sealed.
+
+// Simple extension.
+class SimpleExtend extends BaseMixinClass {}
+//    ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtend extends BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Extending with a sealed class (valid, used to check the errors below).
+sealed class SealedExtend extends BaseMixinClass {}
+
+// Extending through a sealed class.
+class SimpleSealedExtendExtend extends SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedExtendExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceSealedExtendExtend extends SealedExtend {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedExtendExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing through a sealed class.
+class SimpleSealedExtendImplement implements SealedExtend {}
+//                                           ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base class BaseSealedExtendImplement implements SealedExtend {}
+//                                              ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceSealedExtendImplement implements SealedExtend {}
+//                                                        ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+final class FinalSealedExtendImplement implements SealedExtend {}
+//                                                ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedSealedExtendImplement implements SealedExtend {}
+//                                                  ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+mixin class MixinClassSealedExtendImplement implements SealedExtend {}
+//                                                     ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassSealedExtendImplement implements SealedExtend {}
+//                                                              ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+mixin MixinSealedExtendImplement implements SealedExtend {}
+//                                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinSealedExtendImplement implements SealedExtend {}
+//                                                   ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+// Using a sealed class as an `on` type
+
+mixin MixinSealedExtendOn on SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'MixinSealedExtendOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Extending via an anonymous mixin class.
+class SimpleExtendWith extends BaseMixinClass with _MixinOnObject {}
+//    ^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtendWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtendWith extends BaseMixinClass
+//              ^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtendWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject {}
+
+// Extending via an anonymous mixin application class.
+class SimpleExtendApplication = BaseMixinClass with _MixinOnObject;
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtendApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtendApplication = BaseMixinClass with _MixinOnObject;
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtendApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+/// It is an error if BaseMixinClass is implemented by something which is not base,
+/// final or sealed.
+
+// Simple implementation.
+class SimpleImplement implements BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+//                               ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base class BaseImplement implements BaseMixinClass {}
+//                                  ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceImplement implements BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+//                                            ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+final class FinalImplement implements BaseMixinClass {}
+//                                    ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedImplement implements BaseMixinClass {}
+//                                      ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+// Implementing with a mixin class.
+mixin class SimpleMixinClassImplement implements BaseMixinClass {}
+//          ^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+//                                               ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassImplement implements BaseMixinClass {}
+//                                                  ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+// Implementing with a mixin application class.
+class SimpleImplementApplication = Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixinClass;
+//      ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base class BaseImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+//             ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceImplementApplication = Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixinClass;
+//      ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+final class FinalImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+//             ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+//             ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+// Implementing with a mixin.
+mixin SimpleMixinImplement implements BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+//                                    ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinImplement implements BaseMixinClass {}
+//                                       ^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+/// It is an error if BaseMixinClass is the `on` type of something which is not base.
+
+mixin SimpleMixinOn on BaseMixinClass {}
+//    ^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin SimpleMixinOnSimpleBase on SimpleClass, BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin SimpleMixinOnBaseSimple on BaseMixinClass, SimpleClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+/// It is an error to use BaseMixinClass as a mixin, if the result is not base,
+/// final or sealed.
+
+class SimpleMixinClassApply extends Object with BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+class SimpleMixinClassApplySimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+class SimpleMixinClassApplyBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApply extends Object with BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceMixinClassApplySimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplyBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+// Sealed class from mixing in a base mixin class.  (valid, used for tests
+// below)
+sealed class SealedMixinClassApply extends Object with BaseMixinClass {}
+
+// Implementing through a sealed class.
+class SimpleSealedMixinClassApplyImplement implements SealedMixinClassApply {}
+//                                                    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base class BaseSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+final class FinalSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+/// It is an error to use BaseMixinClass as a mixin application, if the result
+/// is not base, final or sealed.
+
+class SimpleMixinClassApplication extends Object with BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+class SimpleMixinClassApplicationSimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+class SimpleMixinClassApplicationBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApplication extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplicationSimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplicationBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+// Sealed class from mixing in a base mixin class.  (valid, used for tests
+// below)
+sealed class SealedMixinApplication = Object with BaseMixinClass;
+
+// Implementing through a sealed class.
+class SimpleSealedMixinApplicationImplement implements SealedMixinApplication {}
+//                                                     ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base class BaseSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+final class FinalSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixinClass' can't be implemented outside of its library because it's a base class.

tests/language/class_modifiers/base_transitivity/base_mixin_class_different_library_test.dart

@@ -0,0 +1,97 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the valid uses of a base mixin class defined in a different library
+
+import "shared_library_definitions.dart" show BaseMixinClass;
+
+mixin _MixinOnObject {}
+
+/// BaseMixinClass can be extended, so long as the subtype is base, final
+/// or sealed.
+
+// Simple extension.
+base class BaseExtend extends BaseMixinClass {}
+
+final class FinalExtend extends BaseMixinClass {}
+
+// Extending with a sealed class.
+sealed class SealedExtend extends BaseMixinClass {}
+
+// Extending through a sealed class.
+base class BaseSealedExtendExtend extends SealedExtend {}
+
+final class FinalSealedExtendExtend extends SealedExtend {}
+
+sealed class SealedSealedExtendExtend extends SealedExtend {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedExtendOn on SealedExtend {}
+
+// Extending via an anonymous mixin class.
+base class BaseExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+final class FinalExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+sealed class SealedExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+// Extending via an anonymous mixin application class.
+final class FinalExtendApplication = BaseMixinClass with _MixinOnObject;
+base class BaseExtendApplication = BaseMixinClass with _MixinOnObject;
+sealed class SealedExtendApplication = BaseMixinClass with _MixinOnObject;
+
+/// BaseMixinClass can be an `on` type, so long as the subtype is base.
+
+base mixin BaseMixinOn on BaseMixinClass {}
+
+/// BaseMixinClass can be used as a mixin, so long as the result is base, final,
+/// or sealed.
+
+base class BaseMixinClassApply extends Object with BaseMixinClass {}
+
+final class FinalMixinClassApply extends Object with BaseMixinClass {}
+
+sealed class SealedMixinClassApply extends Object with BaseMixinClass {}
+
+// Extending through a sealed class.
+base class BaseSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplyOn on SealedMixinClassApply {}
+
+/// BaseMixinClass can be used as a mixin application, so long as the result is
+/// base, final, or sealed.
+
+base class BaseMixinApplication = Object with BaseMixinClass;
+
+final class FinalMixinApplication = Object with BaseMixinClass;
+
+sealed class SealedMixinApplication = Object with BaseMixinClass;
+
+// Extending through a sealed class.
+base class BaseSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationExtend
+    extends SealedMixinApplication {}
+
+// Using a sealed class as an `on` type
+base mixin BaseMixinSealedMixinApplicationOn on SealedMixinApplication {}
+
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/base_mixin_class_same_library_error_test.dart

@@ -0,0 +1,304 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the invalid uses of a base mixin class within the same library
+
+class SimpleClass {}
+
+base mixin class BaseMixinClass {}
+
+mixin _MixinOnObject {}
+
+/// It is an error if BaseMixinClass is extended by something which is not base,
+/// final or sealed.
+
+// Simple extension.
+class SimpleExtend extends BaseMixinClass {}
+//    ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtend extends BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Extending with a sealed class (valid, used to check the errors below).
+sealed class SealedExtend extends BaseMixinClass {}
+
+// Extending through a sealed class.
+class SimpleSealedExtendExtend extends SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedExtendExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceSealedExtendExtend extends SealedExtend {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedExtendExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing through a sealed class.
+class SimpleSealedExtendImplement implements SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceSealedExtendImplement implements SealedExtend {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin class MixinClassSealedExtendImplement implements SealedExtend {}
+//          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'MixinClassSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin MixinSealedExtendImplement implements SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'MixinSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Using a sealed class as an `on` type
+mixin MixinSealedExtendOn on SealedExtend {}
+//    ^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'MixinSealedExtendOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Extending via an anonymous mixin class.
+class SimpleExtendWith extends BaseMixinClass with _MixinOnObject {}
+//    ^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtendWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtendWith extends BaseMixinClass
+//              ^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtendWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject {}
+
+// Extending via an anonymous mixin application class.
+class SimpleExtendApplication = BaseMixinClass with _MixinOnObject;
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleExtendApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceExtendApplication = BaseMixinClass with _MixinOnObject;
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceExtendApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+/// It is an error if BaseMixinClass is implemented by something which is not base,
+/// final or sealed.
+
+// Simple implementation.
+class SimpleImplement implements BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceImplement implements BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing with a sealed class (valid, used for tests below).
+sealed class SealedImplement implements BaseMixinClass {}
+
+// Extending through a sealed class.
+class SimpleSealedImplementExtend extends SealedImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedImplementExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceSealedImplementExtend extends SealedImplement {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedImplementExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing through a sealed class.
+class SimpleSealedImplementImplement implements SealedImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedImplementImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceSealedImplementImplement implements SealedImplement {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedImplementImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing with a mixin class.
+mixin class SimpleMixinClassImplement implements BaseMixinClass {}
+//          ^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing with a base mixin class (valid, used for tests below)
+base mixin class BaseMixinClassImplement implements BaseMixinClass {}
+
+// Implementing by applying a mixin class.
+class SimpleMixinClassImplementApplied extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClassImplement' is 'base'.
+    with
+        BaseMixinClassImplement {}
+
+interface class InterfaceMixinClassImplementApplied extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClassImplement' is 'base'.
+    with
+        BaseMixinClassImplement {}
+
+// Implementing with a mixin application class.
+interface class InterfaceImplementApplication = Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixinClass;
+class SimpleImplementApplication = Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixinClass;
+
+// Implementing with a mixin.
+mixin SimpleMixinImplement implements BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+// Implementing with a base mixin (valid, used for tests below)
+base mixin BaseMixinImplement implements BaseMixinClass {}
+
+// Implementing by applying a mixin.
+class SimpleMixinImplementApplied extends Object with BaseMixinImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinImplement' is 'base'.
+
+interface class InterfaceMixinImplementApplied extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinImplement' is 'base'.
+    with
+        BaseMixinImplement {}
+
+/// It is an error if BaseMixinClass is the `on` type of something which is not base.
+
+mixin SimpleMixinOn on BaseMixinClass {}
+//    ^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin SimpleMixinOnBaseSimple on BaseMixinClass, SimpleClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+mixin SimpleMixinOnSimpleBase on SimpleClass, BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+/// It is an error to use BaseMixinClass as a mixin, if the result is not base,
+/// final or sealed.
+
+class SimpleMixinClassApply extends Object with BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+class SimpleMixinClassApplySimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+class SimpleMixinClassApplyBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApply extends Object with BaseMixinClass {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+interface class InterfaceMixinClassApplySimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplyBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+/// It is an error to use BaseMixinClass as a mixin application, if the result
+/// is not base, final or sealed.
+
+class SimpleMixinClassApplication extends Object with BaseMixinClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+
+class SimpleMixinClassApplicationSimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+class SimpleMixinClassApplicationBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApplication extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplicationSimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixinClass {}
+
+interface class InterfaceMixinClassApplicationBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClass' is 'base'.
+    with
+        BaseMixinClass,
+        _MixinOnObject {}

tests/language/class_modifiers/base_transitivity/base_mixin_class_same_library_test.dart

@@ -0,0 +1,201 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the valid uses of a base mixin class within the same library
+
+base mixin class BaseMixinClass {}
+
+mixin _MixinOnObject {}
+
+/// BaseMixinClass can be extended, so long as the subtype is base, final
+/// or sealed.
+
+// Simple extension.
+base class BaseExtend extends BaseMixinClass {}
+
+final class FinalExtend extends BaseMixinClass {}
+
+// Extending with a sealed class.
+sealed class SealedExtend extends BaseMixinClass {}
+
+// Extending through a sealed class.
+base class BaseSealedExtendExtend extends SealedExtend {}
+
+final class FinalSealedExtendExtend extends SealedExtend {}
+
+sealed class SealedSealedExtendExtend extends SealedExtend {}
+
+// Implementing through a sealed class.
+base class BaseSealedExtendImplement implements SealedExtend {}
+
+final class FinalSealedExtendImplement implements SealedExtend {}
+
+sealed class SealedSealedExtendImplement implements SealedExtend {}
+
+base mixin class BaseMixinClassSealedExtendImplement implements SealedExtend {}
+
+base mixin BaseMixinSealedExtendImplement implements SealedExtend {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedExtendOn on SealedExtend {}
+
+// Extending via an anonymous mixin class.
+base class BaseExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+final class FinalExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+sealed class SealedExtendWith extends BaseMixinClass with _MixinOnObject {}
+
+// Extending via an anonymous mixin application class.
+final class FinalExtendApplication = BaseMixinClass with _MixinOnObject;
+base class BaseExtendApplication = BaseMixinClass with _MixinOnObject;
+sealed class SealedExtendApplication = BaseMixinClass with _MixinOnObject;
+
+/// BaseMixinClass can be implemented, so long as the subtype is base, final, or
+/// sealed
+
+// Simple implementation.
+base class BaseImplement implements BaseMixinClass {}
+
+final class FinalImplement implements BaseMixinClass {}
+
+// Implementing with a sealed class.
+sealed class SealedImplement implements BaseMixinClass {}
+
+// Extending through a sealed class.
+base class BaseSealedImplementExtend extends SealedImplement {}
+
+final class FinalSealedImplementExtend extends SealedImplement {}
+
+sealed class SealedSealedImplementExtend extends SealedImplement {}
+
+// Implementing through a sealed class.
+base class BaseSealedImplementImplement implements SealedImplement {}
+
+final class FinalSealedImplementImplement implements SealedImplement {}
+
+sealed class SealedSealedImplementImplement implements SealedImplement {}
+
+// Implementing with a mixin class.
+base mixin class BaseMixinClassImplement implements BaseMixinClass {}
+
+// Implementing by applying a mixin class.
+base class BaseMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+final class FinalMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+sealed class SealedMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+// Implementing with a mixin application class.
+base class BaseImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+final class FinalImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+sealed class SealedImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixinClass;
+
+// Implementing with a mixin.
+base mixin BaseMixinImplement implements BaseMixinClass {}
+
+// Implementing by applying a mixin.
+base class BaseMixinImplementApplied extends Object with BaseMixinImplement {}
+
+final class FinalMixinImplementApplied extends Object with BaseMixinImplement {}
+
+sealed class SealedMixinImplementApplied extends Object
+    with BaseMixinImplement {}
+
+/// BaseMixinClass can be an `on` type, so long as the subtype is base.
+
+base mixin BaseMixinOn on BaseMixinClass {}
+
+/// BaseMixinClass can be used as a mixin, so long as the result is base, final,
+/// or sealed.
+
+base class BaseMixinClassApply extends Object with BaseMixinClass {}
+
+final class FinalMixinClassApply extends Object with BaseMixinClass {}
+
+sealed class SealedMixinClassApply extends Object with BaseMixinClass {}
+
+// Extending through a sealed class.
+base class BaseSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+// Implementing through a sealed class.
+base class BaseSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+base mixin class BaseMixinClassSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+base mixin BaseMixinSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplyOn on SealedMixinClassApply {}
+
+/// BaseMixinClass can be used as a mixin application, so long as the result is
+/// base, final, or sealed.
+
+base class BaseMixinApplication = Object with BaseMixinClass;
+
+final class FinalMixinApplication = Object with BaseMixinClass;
+
+sealed class SealedMixinApplication = Object with BaseMixinClass;
+
+// Extending through a sealed class.
+base class BaseSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationExtend
+    extends SealedMixinApplication {}
+
+// Implementing through a sealed class.
+base class BaseSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+base mixin class BaseMixinClassSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+base mixin BaseMixinSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplicationOn on SealedMixinApplication {}
+
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/base_mixin_different_library_error_test.dart

@@ -0,0 +1,317 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the invalid uses of a base mixin defined in a different library.
+
+import "shared_library_definitions.dart" show SimpleClass, BaseMixin;
+
+mixin _MixinOnObject {}
+
+/// It is an error if BaseMixin is implemented by something which is not base,
+/// final or sealed.
+
+// Simple implementation.
+class SimpleImplement implements BaseMixin {}
+//    ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+//                               ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+base class BaseImplement implements BaseMixin {}
+//                                  ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+interface class InterfaceImplement implements BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+//                                            ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+final class FinalImplement implements BaseMixin {}
+//                                    ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+sealed class SealedImplement implements BaseMixin {}
+//                                      ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+// Implementing with a mixin class.
+mixin class SimpleMixinClassImplement implements BaseMixin {}
+//          ^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+//                                               ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+base mixin class BaseMixinClassImplement implements BaseMixin {}
+//                                                  ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+// Implementing with a mixin application class.
+class SimpleImplementApplication = Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixin;
+//      ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+base class BaseImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+//             ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+interface class InterfaceImplementApplication = Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixin;
+//      ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+final class FinalImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+//             ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+sealed class SealedImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+//             ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+// Implementing with a mixin.
+mixin SimpleMixinImplement implements BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+//                                    ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+base mixin BaseMixinImplement implements BaseMixin {}
+//                                       ^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
+
+/// It is an error if BaseMixin is the `on` type of something which is not base.
+
+mixin SimpleMixinOn on BaseMixin {}
+//    ^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+mixin SimpleMixinOnSimpleBase on SimpleClass, BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+mixin SimpleMixinOnBaseSimple on BaseMixin, SimpleClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+/// It is an error to use BaseMixin as a mixin, if the result is not base,
+/// final or sealed.
+
+class SimpleMixinClassApply extends Object with BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+class SimpleMixinClassApplySimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+class SimpleMixinClassApplyBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApply extends Object with BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceMixinClassApplySimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+interface class InterfaceMixinClassApplyBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+// Sealed class produced from a base mixin (valid, used for tests below)
+sealed class SealedMixinClassApply extends Object with BaseMixin {}
+
+// Implementing through a sealed class.
+class SimpleSealedMixinClassApplyImplement implements SealedMixinClassApply {}
+//                                                    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base class BaseSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+final class FinalSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+//             ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+/// It is an error to use BaseMixin as a mixin application, if the result
+/// is not base, final or sealed.
+
+class SimpleMixinClassApplication extends Object with BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+class SimpleMixinClassApplicationSimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+class SimpleMixinClassApplicationBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApplication extends Object with BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceMixinClassApplicationSimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+interface class InterfaceMixinClassApplicationBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+// Sealed class produced from a base mixin (valid, used for tests below).
+sealed class SealedMixinApplication = Object with BaseMixin;
+
+// Implementing through a sealed class.
+class SimpleSealedMixinApplicationImplement implements SealedMixinApplication {}
+//                                                     ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base class BaseSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+interface class InterfaceSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+final class FinalSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+sealed class SealedSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base mixin class BaseMixinClassSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.
+
+base mixin BaseMixinSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+//             ^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseMixin' can't be implemented outside of its library because it's a base class.

tests/language/class_modifiers/base_transitivity/base_mixin_different_library_test.dart

@@ -0,0 +1,62 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the valid uses of a base mixin defined in a different library
+
+import "shared_library_definitions.dart" show BaseMixin;
+
+/// BaseMixin can be an `on` type, so long as the subtype is base.
+
+base mixin BaseMixinOn on BaseMixin {}
+
+/// BaseMixin can be used as a mixin, so long as the result is base, final,
+/// or sealed.
+
+base class BaseMixinApply extends Object with BaseMixin {}
+
+final class FinalMixinClassApply extends Object with BaseMixin {}
+
+sealed class SealedMixinClassApply extends Object with BaseMixin {}
+
+// Extending through a sealed class.
+base class BaseSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplyOn on SealedMixinClassApply {}
+
+/// BaseMixin can be used as a mixin application, so long as the result is
+/// base, final, or sealed.
+
+base class BaseMixinApplication = Object with BaseMixin;
+
+final class FinalMixinApplication = Object with BaseMixin;
+
+sealed class SealedMixinApplication = Object with BaseMixin;
+
+// Extending through a sealed class.
+base class BaseSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationExtend
+    extends SealedMixinApplication {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplicationOn on SealedMixinApplication {}
+
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/base_mixin_same_library_error_test.dart

@@ -0,0 +1,223 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the invalid uses of a base mixin class within the same library
+
+class SimpleClass {}
+
+base mixin BaseMixin {}
+
+mixin _MixinOnObject {}
+
+/// It is an error if BaseMixin is implemented by something which is not
+/// base, final or sealed.
+
+// Simple implementation.
+class SimpleImplement implements BaseMixin {}
+//    ^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceImplement implements BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+// Implementing with a sealed class (valid, used for tests below).
+sealed class SealedImplement implements BaseMixin {}
+
+// Extending through a sealed class.
+class SimpleSealedImplementExtend extends SealedImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedImplementExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceSealedImplementExtend extends SealedImplement {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedImplementExtend' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+// Implementing through a sealed class.
+class SimpleSealedImplementImplement implements SealedImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleSealedImplementImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceSealedImplementImplement implements SealedImplement {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceSealedImplementImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+// Implementing with a mixin class.
+mixin class SimpleMixinClassImplement implements BaseMixin {}
+//          ^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+// Implementing with a base mixin class (valid, used for tests below)
+base mixin class BaseMixinClassImplement implements BaseMixin {}
+
+// Implementing by applying a mixin class.
+class SimpleMixinClassImplementApplied extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClassImplement' is 'base'.
+    with
+        BaseMixinClassImplement {}
+
+interface class InterfaceMixinClassImplementApplied extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinClassImplement' is 'base'.
+    with
+        BaseMixinClassImplement {}
+
+// Implementing with a mixin application class.
+interface class InterfaceImplementApplication = Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixin;
+class SimpleImplementApplication = Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleImplementApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject
+    implements
+        BaseMixin;
+
+// Implementing with a mixin.
+mixin SimpleMixinImplement implements BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+// Implementing with a base mixin (valid, used for tests below)
+base mixin BaseMixinImplement implements BaseMixin {}
+
+// Implementing by applying a mixin.
+class SimpleMixinImplementApplied extends Object with BaseMixinImplement {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinImplement' is 'base'.
+
+interface class InterfaceMixinImplementApplied extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinImplementApplied' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixinImplement' is 'base'.
+    with
+        BaseMixinImplement {}
+
+/// It is an error if BaseMixin is the `on` type of something which is not base.
+
+mixin SimpleMixinOn on BaseMixin {}
+//    ^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOn' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+mixin SimpleMixinOnBaseSimple on BaseMixin, SimpleClass {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+mixin SimpleMixinOnSimpleBase on SimpleClass, BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinOnSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+/// It is an error to use BaseMixin as a mixin, if the result is not base,
+/// final or sealed.
+
+class SimpleMixinClassApply extends Object with BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+class SimpleMixinClassApplySimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+class SimpleMixinClassApplyBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApply extends Object with BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApply' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceMixinClassApplySimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplySimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+interface class InterfaceMixinClassApplyBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplyBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+/// It is an error to use BaseMixin as a mixin application, if the result
+/// is not base, final or sealed.
+
+class SimpleMixinClassApplication extends Object with BaseMixin {}
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+class SimpleMixinClassApplicationSimpleBase extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+class SimpleMixinClassApplicationBaseSimple extends Object
+//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'SimpleMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}
+
+interface class InterfaceMixinClassApplication extends Object with BaseMixin {}
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplication' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+
+interface class InterfaceMixinClassApplicationSimpleBase extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationSimpleBase' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        _MixinOnObject,
+        BaseMixin {}
+
+interface class InterfaceMixinClassApplicationBaseSimple extends Object
+//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+// [cfe] The type 'InterfaceMixinClassApplicationBaseSimple' must be 'base', 'final' or 'sealed' because the supertype 'BaseMixin' is 'base'.
+    with
+        BaseMixin,
+        _MixinOnObject {}

tests/language/class_modifiers/base_transitivity/base_mixin_same_library_test.dart

@@ -0,0 +1,156 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+/// Test the valid uses of a base mixin within the same library
+
+base mixin BaseMixin {}
+
+mixin _MixinOnObject {}
+
+/// BaseMixin can be implemented, so long as the subtype is base, final, or
+/// sealed
+
+// Simple implementation.
+base class BaseImplement implements BaseMixin {}
+
+final class FinalImplement implements BaseMixin {}
+
+// Implementing with a sealed class.
+sealed class SealedImplement implements BaseMixin {}
+
+// Extending through a sealed class.
+base class BaseSealedImplementExtend extends SealedImplement {}
+
+final class FinalSealedImplementExtend extends SealedImplement {}
+
+sealed class SealedSealedImplementExtend extends SealedImplement {}
+
+// Implementing through a sealed class.
+base class BaseSealedImplementImplement implements SealedImplement {}
+
+final class FinalSealedImplementImplement implements SealedImplement {}
+
+sealed class SealedSealedImplementImplement implements SealedImplement {}
+
+// Implementing with a mixin class.
+base mixin class BaseMixinClassImplement implements BaseMixin {}
+
+// Implementing by applying a mixin class.
+base class BaseMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+final class FinalMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+sealed class SealedMixinClassImplementApplied extends Object
+    with BaseMixinClassImplement {}
+
+// Implementing with a mixin application class.
+base class BaseImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+final class FinalImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+sealed class SealedImplementApplication = Object
+    with _MixinOnObject
+    implements BaseMixin;
+
+// Implementing with a mixin.
+base mixin BaseMixinImplement implements BaseMixin {}
+
+// Implementing by applying a mixin.
+base class BaseMixinImplementApplied extends Object with BaseMixinImplement {}
+
+final class FinalMixinImplementApplied extends Object with BaseMixinImplement {}
+
+sealed class SealedMixinImplementApplied extends Object
+    with BaseMixinImplement {}
+
+/// BaseMixin can be an `on` type, so long as the subtype is base.
+
+base mixin BaseMixinOn on BaseMixin {}
+
+/// BaseMixin can be used as a mixin, so long as the result is base, final,
+/// or sealed.
+
+base class BaseMixinApply extends Object with BaseMixin {}
+
+final class FinalMixinClassApply extends Object with BaseMixin {}
+
+sealed class SealedMixinClassApply extends Object with BaseMixin {}
+
+// Extending through a sealed class.
+base class BaseSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyExtend extends SealedMixinClassApply {}
+
+// Implementing through a sealed class.
+base class BaseSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+final class FinalSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+sealed class SealedSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+base mixin class BaseMixinSealedMixinClassApplyImplement
+    implements SealedMixinClassApply {}
+
+base mixin BaseMixinSealedMixinApplyImplement
+    implements SealedMixinClassApply {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplyOn on SealedMixinClassApply {}
+
+/// BaseMixin can be used as a mixin application, so long as the result is
+/// base, final, or sealed.
+
+base class BaseMixinApplication = Object with BaseMixin;
+
+final class FinalMixinApplication = Object with BaseMixin;
+
+sealed class SealedMixinApplication = Object with BaseMixin;
+
+// Extending through a sealed class.
+base class BaseSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationExtend extends SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationExtend
+    extends SealedMixinApplication {}
+
+// Implementing through a sealed class.
+base class BaseSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+final class FinalSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+sealed class SealedSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+base mixin class BaseMixinClassSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+base mixin BaseMixinSealedMixinApplicationImplement
+    implements SealedMixinApplication {}
+
+// Using a sealed class as an `on` type
+
+base mixin BaseMixinSealedMixinApplicationOn on SealedMixinApplication {}
+
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/final_class_different_library_error_test.dart

@@ -197,7 +197,17 @@ mixin SimpleMixinOnSimpleFinal on SimpleClass, FinalClass {}
 
 base mixin BaseMixinOn on FinalClass {}
 // ^
-// [analyzer] unspecified
 // [cfe] unspecified
+// [analyzer] unspecified
+
+base mixin BaseMixinOnFinalSimple on SimpleClass, FinalClass {}
+// ^
+// [cfe] unspecified
+// [analyzer] unspecified
+
+base mixin BaseMixinOnSimpleFinal on FinalClass, SimpleClass {}
+// ^
+// [cfe] unspecified
+// [analyzer] unspecified
 
 main() {}

tests/language/class_modifiers/base_transitivity/final_class_same_library_test.dart

@@ -119,4 +119,10 @@ sealed class SealedMixinImplementApplied extends Object
 
 base mixin BaseMixinOn on FinalClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/interface_class_different_library_test.dart

@@ -113,4 +113,10 @@ mixin SimpleMixinOn on InterfaceClass {}
 
 base mixin BaseMixinOn on InterfaceClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/interface_class_same_library_test.dart

@@ -185,4 +185,10 @@ mixin SimpleMixinOn on InterfaceClass {}
 
 base mixin BaseMixinOn on InterfaceClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.

tests/language/class_modifiers/base_transitivity/sealed_class_same_library_test.dart

@@ -184,4 +184,10 @@ mixin SimpleMixinOn on SealedClass {}
 
 base mixin BaseMixinOn on SealedClass {}
 
-main() {}
+// This test is intended just to check that certain combinations of modifiers
+// are statically allowed.  Make this a static error test so that backends don't
+// try to run it.
+int x = "This is a static error test";
+//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_ASSIGNMENT
+// [cfe] A value of type 'String' can't be assigned to a variable of type 'int'.