[cfe] Report error when using a class as a mixin when sealed-class is enabled.

From the changes in dart-lang/language#2674 that specify from a certain language version, we would like classes to not be used as mixins unless specified as a 'mixin class'.

Currently, this behaviour is under the sealed-class flag. May be subject to change as the other modifiers are added, but I'd at least like to make sure this works for sealed classes.

Change-Id: I5754b383327dde06d49175fe2d05c8ba7462145f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/273082
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -905,6 +905,27 @@ Message _withArgumentsCantReadFile(Uri uri_, String string) {
       arguments: {'uri': uri_, 'string': string});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<Message Function(String name)> templateCantUseClassAsMixin =
+    const Template<Message Function(String name)>(
+        problemMessageTemplate: r"""Class '#name' can't be used as a mixin.""",
+        withArguments: _withArgumentsCantUseClassAsMixin);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Message Function(String name)> codeCantUseClassAsMixin =
+    const Code<Message Function(String name)>(
+  "CantUseClassAsMixin",
+);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsCantUseClassAsMixin(String name) {
+  if (name.isEmpty) throw 'No name provided';
+  name = demangleMixinApplicationName(name);
+  return new Message(codeCantUseClassAsMixin,
+      problemMessage: """Class '${name}' can't be used as a mixin.""",
+      arguments: {'name': name});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<Message Function(Token token)>
     templateCantUseControlFlowOrSpreadAsConstant =

pkg/front_end/lib/src/fasta/source/source_loader.dart

@@ -2069,44 +2069,60 @@ severity: $severity
               noLength);
         }
       } else if (supertype is ClassBuilder &&
-          supertype.isSealed &&
-          supertype.libraryBuilder.origin != cls.libraryBuilder.origin) {
-        // If the class is a mixin declaration with an `on` clause which is the
-        // supertype we are evaluating right now, we want to avoid reporting an
-        // error.
-        bool superTypeIsOnClause = false;
-        if (cls.cls.isAnonymousMixin && !cls.cls.isEliminatedMixin) {
-          // The class has multiple 'on' clauses.
-          List<TypeBuilder>? classInterfaces = cls.interfaceBuilders;
-          if (classInterfaces != null) {
-            for (TypeBuilder interface in classInterfaces) {
-              if (interface.declaration == supertype) {
-                superTypeIsOnClause = true;
-                break;
+          cls.libraryBuilder.libraryFeatures.sealedClass.isEnabled) {
+        // Check for implicit class mixins.
+        TypeBuilder? mixedInType = cls.mixedInTypeBuilder;
+        if (cls.isMixinApplication &&
+            mixedInType != null &&
+            mixedInType.declaration == supertype &&
+            !supertype.isMixinDeclaration) {
+          cls.addProblem(
+              templateCantUseClassAsMixin
+                  .withArguments(supertype.fullNameForErrors),
+              cls.charOffset,
+              noLength);
+        }
+
+        // Report error for subtyping outside of sealed supertype's library.
+        if (supertype.isSealed &&
+            supertype.libraryBuilder.origin != cls.libraryBuilder.origin) {
+          // If the class is a mixin declaration with an `on` clause which is
+          // the supertype we are evaluating right now, we want to avoid
+          // reporting an error.
+          bool superTypeIsOnClause = false;
+          if (cls.cls.isAnonymousMixin && !cls.cls.isEliminatedMixin) {
+            // The class has multiple 'on' clauses.
+            List<TypeBuilder>? classInterfaces = cls.interfaceBuilders;
+            if (classInterfaces != null) {
+              for (TypeBuilder interface in classInterfaces) {
+                if (interface.declaration == supertype) {
+                  superTypeIsOnClause = true;
+                  break;
+                }
               }
             }
+          } else if (cls.isMixinDeclaration) {
+            // The class has up to one 'on' clause.
+            TypeBuilder? classSuperType = cls.supertypeBuilder;
+            if (classSuperType != null &&
+                classSuperType.declaration == supertype) {
+              superTypeIsOnClause = true;
+            }
           }
-        } else if (cls.isMixinDeclaration) {
-          // The class has up to one 'on' clause.
-          TypeBuilder? classSuperType = cls.supertypeBuilder;
-          if (classSuperType != null &&
-              classSuperType.declaration == supertype) {
-            superTypeIsOnClause = true;
-          }
-        }
-        if (!superTypeIsOnClause) {
-          if (supertype.isMixinDeclaration) {
-            cls.addProblem(
-                templateSealedMixinSubtypeOutsideOfLibrary
-                    .withArguments(supertype.fullNameForErrors),
-                cls.charOffset,
-                noLength);
-          } else {
-            cls.addProblem(
-                templateSealedClassSubtypeOutsideOfLibrary
-                    .withArguments(supertype.fullNameForErrors),
-                cls.charOffset,
-                noLength);
+          if (!superTypeIsOnClause) {
+            if (supertype.isMixinDeclaration) {
+              cls.addProblem(
+                  templateSealedMixinSubtypeOutsideOfLibrary
+                      .withArguments(supertype.fullNameForErrors),
+                  cls.charOffset,
+                  noLength);
+            } else {
+              cls.addProblem(
+                  templateSealedClassSubtypeOutsideOfLibrary
+                      .withArguments(supertype.fullNameForErrors),
+                  cls.charOffset,
+                  noLength);
+            }
           }
         }
       }

pkg/front_end/messages.status

@@ -64,6 +64,7 @@ CantInferPackagesFromPackageUri/analyzerCode: Fail
 CantInferPackagesFromPackageUri/example: Fail
 CantInferTypeDueToCircularity/example: Fail
 CantReadFile/part_wrapped_script: Fail # Importing file in the (now) part.
+CantUseClassAsMixin/analyzerCode: Fail
 CantUseControlFlowOrSpreadAsConstant/example: Fail
 CantUseDeferredPrefixAsConstant/part_wrapped_script: Fail # Importing file in the (now) part.
 CantUsePrefixAsExpression/part_wrapped_script: Fail # Importing file in the (now) part.

pkg/front_end/messages.yaml

@@ -6021,3 +6021,10 @@ SealedMixinSubtypeOutsideOfLibrary:
       class B with M {}
     lib.dart:
       sealed mixin M {}
+
+CantUseClassAsMixin:
+  problemMessage: "Class '#name' can't be used as a mixin."
+  experiments: sealed-class
+  script: |
+    class A {}
+    class B with A {}

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.strong.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.strong.transformed.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.weak.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.weak.modular.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.weak.outline.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart.weak.transformed.expect

@@ -10,6 +10,10 @@ library /*isNonNullableByDefault*/;
 // class Class2 implements Sealed {}
 //       ^
 //
+// pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Class 'Sealed' can't be used as a mixin.
+// class Class3 with Sealed {}
+//       ^
+//
 // pkg/front_end/testcases/sealed_class/extends_implements_with/main.dart:11:7: Error: Sealed class 'Sealed' can't be extended, implemented, or mixed in outside of its library.
 // class Class3 with Sealed {}
 //       ^

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart

@@ -0,0 +1,20 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+sealed class SealedClass {}
+class A with SealedClass {}
+
+sealed mixin SealedMixin {}
+class B with SealedClass, SealedMixin {}
+
+class Class {}
+class C with Class, SealedClass {}
+class D with Class, SealedMixin {}
+
+mixin Mixin {}
+class E with Mixin, SealedClass {}
+class F with Mixin, SealedMixin {}
+
+// TODO(kallentu): Move this to a more generic test.
+class G with Class, Mixin {}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.strong.expect

@@ -0,0 +1,149 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _A&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    : super self::_A&Object&SealedClass::•()
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin = self::_B&Object&SealedClass with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    : super self::_B&Object&SealedClass&SealedMixin::•()
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass = self::_C&Object&Class with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    : super self::_C&Object&Class&SealedClass::•()
+    ;
+}
+abstract class _D&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin = self::_D&Object&Class with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    : super self::_D&Object&Class&SealedMixin::•()
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass = self::_E&Object&Mixin with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    : super self::_E&Object&Mixin&SealedClass::•()
+    ;
+}
+abstract class _F&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin = self::_F&Object&Mixin with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    : super self::_F&Object&Mixin&SealedMixin::•()
+    ;
+}
+abstract class _G&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin = self::_G&Object&Class with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    : super self::_G&Object&Class&Mixin::•()
+    ;
+}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.strong.transformed.expect

@@ -0,0 +1,149 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _A&Object&SealedClass extends core::Object implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    : super self::_A&Object&SealedClass::•()
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass extends core::Object implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin extends self::_B&Object&SealedClass implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    : super self::_B&Object&SealedClass&SealedMixin::•()
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass extends self::_C&Object&Class implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    : super self::_C&Object&Class&SealedClass::•()
+    ;
+}
+abstract class _D&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin extends self::_D&Object&Class implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    : super self::_D&Object&Class&SealedMixin::•()
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin extends core::Object implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass extends self::_E&Object&Mixin implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    : super self::_E&Object&Mixin&SealedClass::•()
+    ;
+}
+abstract class _F&Object&Mixin extends core::Object implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin extends self::_F&Object&Mixin implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    : super self::_F&Object&Mixin&SealedMixin::•()
+    ;
+}
+abstract class _G&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin extends self::_G&Object&Class implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    : super self::_G&Object&Class&Mixin::•()
+    ;
+}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.textual_outline.expect

@@ -0,0 +1,13 @@
+sealed
+class SealedClass {}
+class A with SealedClass {}
+sealed
+mixin SealedMixin {}
+class B with SealedClass, SealedMixin {}
+class Class {}
+class C with Class, SealedClass {}
+class D with Class, SealedMixin {}
+mixin Mixin {}
+class E with Mixin, SealedClass {}
+class F with Mixin, SealedMixin {}
+class G with Class, Mixin {}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.weak.expect

@@ -0,0 +1,149 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _A&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    : super self::_A&Object&SealedClass::•()
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin = self::_B&Object&SealedClass with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    : super self::_B&Object&SealedClass&SealedMixin::•()
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass = self::_C&Object&Class with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    : super self::_C&Object&Class&SealedClass::•()
+    ;
+}
+abstract class _D&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin = self::_D&Object&Class with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    : super self::_D&Object&Class&SealedMixin::•()
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass = self::_E&Object&Mixin with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    : super self::_E&Object&Mixin&SealedClass::•()
+    ;
+}
+abstract class _F&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin = self::_F&Object&Mixin with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    : super self::_F&Object&Mixin&SealedMixin::•()
+    ;
+}
+abstract class _G&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin = self::_G&Object&Class with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    : super self::_G&Object&Class&Mixin::•()
+    ;
+}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.weak.modular.expect

@@ -0,0 +1,149 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _A&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    : super self::_A&Object&SealedClass::•()
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin = self::_B&Object&SealedClass with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    : super self::_B&Object&SealedClass&SealedMixin::•()
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass = self::_C&Object&Class with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    : super self::_C&Object&Class&SealedClass::•()
+    ;
+}
+abstract class _D&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin = self::_D&Object&Class with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    : super self::_D&Object&Class&SealedMixin::•()
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass = self::_E&Object&Mixin with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    : super self::_E&Object&Mixin&SealedClass::•()
+    ;
+}
+abstract class _F&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin = self::_F&Object&Mixin with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    : super self::_F&Object&Mixin&SealedMixin::•()
+    ;
+}
+abstract class _G&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin = self::_G&Object&Class with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    : super self::_G&Object&Class&Mixin::•()
+    ;
+}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.weak.outline.expect

@@ -0,0 +1,140 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    ;
+}
+abstract class _A&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass = core::Object with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin = self::_B&Object&SealedClass with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    ;
+}
+abstract class _C&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass = self::_C&Object&Class with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    ;
+}
+abstract class _D&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin = self::_D&Object&Class with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass = self::_E&Object&Mixin with self::SealedClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    ;
+}
+abstract class _F&Object&Mixin = core::Object with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin = self::_F&Object&Mixin with self::SealedMixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    ;
+}
+abstract class _G&Object&Class = core::Object with self::Class /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin = self::_G&Object&Class with self::Mixin /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    ;
+}

pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart.weak.transformed.expect

@@ -0,0 +1,149 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:6:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class A with SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:9:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class B with SealedClass, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'Class' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:12:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class C with Class, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:13:7: Error: Class 'Class' can't be used as a mixin.
+// class D with Class, SealedMixin {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:16:7: Error: Class 'SealedClass' can't be used as a mixin.
+// class E with Mixin, SealedClass {}
+//       ^
+//
+// pkg/front_end/testcases/sealed_class/sealed_class_as_mixin.dart:20:7: Error: Class 'Class' can't be used as a mixin.
+// class G with Class, Mixin {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+abstract sealed class SealedClass extends core::Object {
+  synthetic constructor •() → self::SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _A&Object&SealedClass extends core::Object implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_A&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+class A extends self::_A&Object&SealedClass {
+  synthetic constructor •() → self::A
+    : super self::_A&Object&SealedClass::•()
+    ;
+}
+abstract sealed class SealedMixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _B&Object&SealedClass extends core::Object implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass
+    : super core::Object::•()
+    ;
+}
+abstract class _B&Object&SealedClass&SealedMixin extends self::_B&Object&SealedClass implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_B&Object&SealedClass&SealedMixin
+    : super self::_B&Object&SealedClass::•()
+    ;
+}
+class B extends self::_B&Object&SealedClass&SealedMixin {
+  synthetic constructor •() → self::B
+    : super self::_B&Object&SealedClass&SealedMixin::•()
+    ;
+}
+class Class extends core::Object {
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _C&Object&Class&SealedClass extends self::_C&Object&Class implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_C&Object&Class&SealedClass
+    : super self::_C&Object&Class::•()
+    ;
+}
+class C extends self::_C&Object&Class&SealedClass {
+  synthetic constructor •() → self::C
+    : super self::_C&Object&Class&SealedClass::•()
+    ;
+}
+abstract class _D&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _D&Object&Class&SealedMixin extends self::_D&Object&Class implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_D&Object&Class&SealedMixin
+    : super self::_D&Object&Class::•()
+    ;
+}
+class D extends self::_D&Object&Class&SealedMixin {
+  synthetic constructor •() → self::D
+    : super self::_D&Object&Class&SealedMixin::•()
+    ;
+}
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+}
+abstract class _E&Object&Mixin extends core::Object implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _E&Object&Mixin&SealedClass extends self::_E&Object&Mixin implements self::SealedClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E&Object&Mixin&SealedClass
+    : super self::_E&Object&Mixin::•()
+    ;
+}
+class E extends self::_E&Object&Mixin&SealedClass {
+  synthetic constructor •() → self::E
+    : super self::_E&Object&Mixin&SealedClass::•()
+    ;
+}
+abstract class _F&Object&Mixin extends core::Object implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin
+    : super core::Object::•()
+    ;
+}
+abstract class _F&Object&Mixin&SealedMixin extends self::_F&Object&Mixin implements self::SealedMixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_F&Object&Mixin&SealedMixin
+    : super self::_F&Object&Mixin::•()
+    ;
+}
+class F extends self::_F&Object&Mixin&SealedMixin {
+  synthetic constructor •() → self::F
+    : super self::_F&Object&Mixin&SealedMixin::•()
+    ;
+}
+abstract class _G&Object&Class extends core::Object implements self::Class /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class
+    : super core::Object::•()
+    ;
+}
+abstract class _G&Object&Class&Mixin extends self::_G&Object&Class implements self::Mixin /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_G&Object&Class&Mixin
+    : super self::_G&Object&Class::•()
+    ;
+}
+class G extends self::_G&Object&Class&Mixin {
+  synthetic constructor •() → self::G
+    : super self::_G&Object&Class&Mixin::•()
+    ;
+}

pkg/front_end/testcases/textual_outline.status

@@ -202,6 +202,7 @@ regress/issue_39091_1: FormatterCrash
 regress/issue_41265.crash: Crash
 regress/issue_41265.crash: FormatterCrash
 sealed_class/sealed_abstract_class_declaration: FormatterCrash
+sealed_class/sealed_class_as_mixin: FormatterCrash
 sealed_class/sealed_class_declaration: FormatterCrash
 sealed_class/sealed_mixin_declaration: FormatterCrash
 super_parameters/issue47741: FormatterCrash

tests/language/sealed_class/sealed_class_as_mixin_error_test.dart

@@ -6,16 +6,18 @@
 
 // Error when attempting to mix in a sealed class outside of library.
 
-import 'sealed_class_as_mixin_in_lib_error_test.dart';
+import 'sealed_class_as_mixin_lib.dart';
 
 abstract class OutsideA with SealedClass {}
 //             ^
 // [analyzer] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
 // [cfe] Sealed class 'SealedClass' can't be extended, implemented, or mixed in outside of its library.
 
 class OutsideB with SealedClass {
 //    ^
 // [analyzer] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
 // [cfe] Sealed class 'SealedClass' can't be extended, implemented, or mixed in outside of its library.
   @override
   int foo = 2;
@@ -27,4 +29,29 @@ class OutsideB with SealedClass {
 abstract class OutsideC = Object with SealedClass;
 //             ^
 // [analyzer] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
 // [cfe] Sealed class 'SealedClass' can't be extended, implemented, or mixed in outside of its library.
+
+abstract class OutsideD with SealedClass, Class {}
+//             ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.
+// [cfe] Class 'SealedClass' can't be used as a mixin.
+// [cfe] Sealed class 'SealedClass' can't be extended, implemented, or mixed in outside of its library.
+
+class OutsideE with Class, SealedMixin {}
+//    ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.
+// [cfe] Sealed mixin 'SealedMixin' can't be mixed in outside of its library.
+
+abstract class OutsideF with Mixin, SealedClass {}
+//             ^
+// [analyzer] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
+// [cfe] Sealed class 'SealedClass' can't be extended, implemented, or mixed in outside of its library.
+
+class OutsideG with Mixin, Class {}
+//    ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.

tests/language/sealed_class/sealed_class_as_mixin_in_lib_error_test.dart

@@ -12,16 +12,19 @@ sealed class SealedClass {
   int nonAbstractBar(int value) => value + 100;
   int bar(int value);
 }
+sealed mixin SealedMixin {}
+class Class {}
+mixin Mixin {}
 
 abstract class A with SealedClass {}
-// ^
+//             ^
 // [analyzer] unspecified
-// [cfe] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
 
 class B with SealedClass {
-// ^
+//    ^
 // [analyzer] unspecified
-// [cfe] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
   @override
   int nonAbstractFoo = 100;
 
@@ -33,6 +36,27 @@ class B with SealedClass {
 }
 
 abstract class C = Object with SealedClass;
-// ^
+//             ^
 // [analyzer] unspecified
-// [cfe] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
+
+abstract class D with SealedClass, Class {}
+//             ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.
+// [cfe] Class 'SealedClass' can't be used as a mixin.
+
+class E with Class, SealedMixin {}
+//    ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.
+
+abstract class F with Mixin, SealedClass {}
+//             ^
+// [analyzer] unspecified
+// [cfe] Class 'SealedClass' can't be used as a mixin.
+
+class G with Mixin, Class {}
+//    ^
+// [analyzer] unspecified
+// [cfe] Class 'Class' can't be used as a mixin.

tests/language/sealed_class/sealed_class_as_mixin_lib.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// SharedOptions=--enable-experiment=sealed-class
+
+sealed class SealedClass {
+  int nonAbstractFoo = 0;
+  abstract int foo;
+  int nonAbstractBar(int value) => value + 100;
+  int bar(int value);
+}
+sealed mixin SealedMixin {}
+class Class {}
+mixin Mixin {}