[analyzer/cfe] Report an error when declaring a mixin class with final, interface, or sealed.

Change-Id: Ia1393851dffaab55c31d04d4e81bfae83a7bd67f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283126
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

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

@@ -5296,6 +5296,15 @@ Message _withArgumentsFinalFieldWithoutInitializer(String name) {
       arguments: {'name': name});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeFinalMixinClass = messageFinalMixinClass;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageFinalMixinClass = const MessageCode("FinalMixinClass",
+    index: 142,
+    problemMessage: r"""A mixin class can't be declared 'final'.""",
+    correctionMessage: r"""Try removing the 'final' keyword.""");
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
     Message Function(
@@ -6418,6 +6427,16 @@ Message _withArgumentsInterfaceClassExtendedOutsideOfLibrary(String name) {
       arguments: {'name': name});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeInterfaceMixinClass = messageInterfaceMixinClass;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageInterfaceMixinClass = const MessageCode(
+    "InterfaceMixinClass",
+    index: 143,
+    problemMessage: r"""A mixin class can't be declared 'interface'.""",
+    correctionMessage: r"""Try removing the 'interface' keyword.""");
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
     Message Function(
@@ -11306,6 +11325,16 @@ Message _withArgumentsSealedClassSubtypeOutsideOfLibrary(String name) {
       arguments: {'name': name});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeSealedMixinClass = messageSealedMixinClass;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageSealedMixinClass = const MessageCode(
+    "SealedMixinClass",
+    index: 144,
+    problemMessage: r"""A mixin class can't be declared 'sealed'.""",
+    correctionMessage: r"""Try removing the 'sealed' keyword.""");
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
     Message Function(

pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart

@@ -568,7 +568,7 @@ class Parser {
         optional('class', next.next!)) {
       macroToken = next;
       next = next.next!;
-    } else if (next.isIdentifier && next.lexeme == 'sealed') {
+    } else if (next.isIdentifier && optional('sealed', next)) {
       sealedToken = next;
       if (optional('class', next.next!) || optional('mixin', next.next!)) {
         next = next.next!;
@@ -579,12 +579,12 @@ class Parser {
         start = next;
         next = next.next!.next!;
       }
-    } else if (next.isIdentifier && next.lexeme == 'base') {
+    } else if (next.isIdentifier && optional('base', next)) {
       baseToken = next;
       if (optional('class', next.next!) || optional('mixin', next.next!)) {
         next = next.next!;
       }
-    } else if (next.isIdentifier && next.lexeme == 'interface') {
+    } else if (next.isIdentifier && optional('interface', next)) {
       interfaceToken = next;
       if (optional('class', next.next!) || optional('mixin', next.next!)) {
         next = next.next!;
@@ -712,8 +712,6 @@ class Parser {
           return parseTypedef(keyword);
         } else if (identical(value, 'mixin')) {
           if (identical(nextValue, 'class')) {
-            // TODO(kallentu): Error handling for any class modifier here other
-            // than base. Only base mixin classes are allowed.
             return _handleModifiersForClassDeclaration(
                 start,
                 keyword.next!,
@@ -721,7 +719,7 @@ class Parser {
                 inlineToken,
                 sealedToken,
                 baseToken,
-                /* interfaceToken = */ null,
+                interfaceToken,
                 keyword,
                 directiveState);
           }
@@ -767,6 +765,19 @@ class Parser {
     ModifierContext context = new ModifierContext(this);
     if (mixinToken != null) {
       context.parseClassModifiers(start, mixinToken);
+
+      // Mixin classes can't have any modifier other than a base modifier.
+      if (context.finalToken != null) {
+        reportRecoverableError(
+            context.finalToken!, codes.messageFinalMixinClass);
+      }
+      if (interfaceToken != null) {
+        reportRecoverableError(
+            interfaceToken, codes.messageInterfaceMixinClass);
+      }
+      if (sealedToken != null) {
+        reportRecoverableError(sealedToken, codes.messageSealedMixinClass);
+      }
     } else {
       context.parseClassModifiers(start, classKeyword);
     }

pkg/analysis_server/lib/src/services/correction/error_fix_status.yaml

@@ -2247,6 +2247,8 @@ ParserErrorCode.FINAL_ENUM:
   status: needsEvaluation
 ParserErrorCode.FINAL_METHOD:
   status: needsEvaluation
+ParserErrorCode.FINAL_MIXIN_CLASS:
+  status: needsEvaluation
 ParserErrorCode.FINAL_TYPEDEF:
   status: needsEvaluation
 ParserErrorCode.FUNCTION_TYPED_PARAMETER_VAR:
@@ -2269,6 +2271,8 @@ ParserErrorCode.IMPORT_DIRECTIVE_AFTER_PART_DIRECTIVE:
   status: needsEvaluation
 ParserErrorCode.INITIALIZED_VARIABLE_IN_FOR_EACH:
   status: needsEvaluation
+ParserErrorCode.INTERFACE_MIXIN_CLASS:
+  status: needsEvaluation
 ParserErrorCode.INVALID_AWAIT_IN_FOR:
   status: needsEvaluation
 ParserErrorCode.INVALID_CODE_POINT:
@@ -2455,6 +2459,8 @@ ParserErrorCode.REDIRECTING_CONSTRUCTOR_WITH_BODY:
   status: needsEvaluation
 ParserErrorCode.REDIRECTION_IN_NON_FACTORY_CONSTRUCTOR:
   status: needsEvaluation
+ParserErrorCode.SEALED_MIXIN_CLASS:
+  status: needsEvaluation
 ParserErrorCode.SETTER_CONSTRUCTOR:
   status: needsEvaluation
 ParserErrorCode.SETTER_IN_FUNCTION:

pkg/analyzer/lib/src/dart/error/syntactic_errors.g.dart

@@ -156,6 +156,9 @@ final fastaAnalyzerErrorCodes = <ErrorCode?>[
   ParserErrorCode.INVALID_CONSTANT_PATTERN_GENERIC,
   ParserErrorCode.INVALID_CONSTANT_CONST_PREFIX,
   ParserErrorCode.INVALID_CONSTANT_PATTERN_BINARY,
+  ParserErrorCode.FINAL_MIXIN_CLASS,
+  ParserErrorCode.INTERFACE_MIXIN_CLASS,
+  ParserErrorCode.SEALED_MIXIN_CLASS,
 ];
 
 class ParserErrorCode extends ErrorCode {
@@ -820,6 +823,12 @@ class ParserErrorCode extends ErrorCode {
     correctionMessage: "Try removing the keyword 'final'.",
   );
 
+  static const ParserErrorCode FINAL_MIXIN_CLASS = ParserErrorCode(
+    'FINAL_MIXIN_CLASS',
+    "A mixin class can't be declared 'final'.",
+    correctionMessage: "Try removing the 'final' keyword.",
+  );
+
   static const ParserErrorCode FINAL_TYPEDEF = ParserErrorCode(
     'FINAL_TYPEDEF',
     "Typedefs can't be declared to be 'final'.",
@@ -897,6 +906,12 @@ class ParserErrorCode extends ErrorCode {
         "Try removing the initializer, or using a different kind of loop.",
   );
 
+  static const ParserErrorCode INTERFACE_MIXIN_CLASS = ParserErrorCode(
+    'INTERFACE_MIXIN_CLASS',
+    "A mixin class can't be declared 'interface'.",
+    correctionMessage: "Try removing the 'interface' keyword.",
+  );
+
   static const ParserErrorCode INVALID_AWAIT_IN_FOR = ParserErrorCode(
     'INVALID_AWAIT_IN_FOR',
     "The keyword 'await' isn't allowed for a normal 'for' statement.",
@@ -1544,6 +1559,12 @@ class ParserErrorCode extends ErrorCode {
         "Try making this a factory constructor, or remove the redirection.",
   );
 
+  static const ParserErrorCode SEALED_MIXIN_CLASS = ParserErrorCode(
+    'SEALED_MIXIN_CLASS',
+    "A mixin class can't be declared 'sealed'.",
+    correctionMessage: "Try removing the 'sealed' keyword.",
+  );
+
   static const ParserErrorCode SETTER_CONSTRUCTOR = ParserErrorCode(
     'SETTER_CONSTRUCTOR',
     "Constructors can't be a setter.",

pkg/analyzer/lib/src/error/error_code_values.g.dart

@@ -742,6 +742,7 @@ const List<ErrorCode> errorCodeValues = [
   ParserErrorCode.FINAL_CONSTRUCTOR,
   ParserErrorCode.FINAL_ENUM,
   ParserErrorCode.FINAL_METHOD,
+  ParserErrorCode.FINAL_MIXIN_CLASS,
   ParserErrorCode.FINAL_TYPEDEF,
   ParserErrorCode.FUNCTION_TYPED_PARAMETER_VAR,
   ParserErrorCode.GETTER_CONSTRUCTOR,
@@ -753,6 +754,7 @@ const List<ErrorCode> errorCodeValues = [
   ParserErrorCode.IMPLEMENTS_BEFORE_WITH,
   ParserErrorCode.IMPORT_DIRECTIVE_AFTER_PART_DIRECTIVE,
   ParserErrorCode.INITIALIZED_VARIABLE_IN_FOR_EACH,
+  ParserErrorCode.INTERFACE_MIXIN_CLASS,
   ParserErrorCode.INVALID_AWAIT_IN_FOR,
   ParserErrorCode.INVALID_CODE_POINT,
   ParserErrorCode.INVALID_COMMENT_REFERENCE,
@@ -846,6 +848,7 @@ const List<ErrorCode> errorCodeValues = [
   ParserErrorCode.RECORD_TYPE_ONE_POSITIONAL_NO_TRAILING_COMMA,
   ParserErrorCode.REDIRECTING_CONSTRUCTOR_WITH_BODY,
   ParserErrorCode.REDIRECTION_IN_NON_FACTORY_CONSTRUCTOR,
+  ParserErrorCode.SEALED_MIXIN_CLASS,
   ParserErrorCode.SETTER_CONSTRUCTOR,
   ParserErrorCode.SETTER_IN_FUNCTION,
   ParserErrorCode.STACK_OVERFLOW,

pkg/analyzer/test/src/fasta/ast_builder_test.dart

@@ -284,6 +284,26 @@ ClassDeclaration
 ''');
   }
 
+  void test_class_final_mixin() {
+    var parseResult = parseStringWithErrors(r'''
+final mixin class A {}
+''');
+    parseResult.assertErrors([
+      error(ParserErrorCode.FINAL_MIXIN_CLASS, 0, 5),
+    ]);
+
+    var node = parseResult.findNode.classDeclaration('class A {}');
+    assertParsedNodeText(node, r'''
+ClassDeclaration
+  finalKeyword: final
+  mixinKeyword: mixin
+  classKeyword: class
+  name: A
+  leftBracket: {
+  rightBracket: }
+''');
+  }
+
   void test_class_implementsClause_recordType() {
     var parseResult = parseStringWithErrors(r'''
 class C implements A, (int, int), B {}
@@ -348,6 +368,26 @@ ClassDeclaration
 ''');
   }
 
+  void test_class_interface_mixin() {
+    var parseResult = parseStringWithErrors(r'''
+interface mixin class A {}
+''');
+    parseResult.assertErrors([
+      error(ParserErrorCode.INTERFACE_MIXIN_CLASS, 0, 9),
+    ]);
+
+    var node = parseResult.findNode.classDeclaration('class A {}');
+    assertParsedNodeText(node, r'''
+ClassDeclaration
+  interfaceKeyword: interface
+  mixinKeyword: mixin
+  classKeyword: class
+  name: A
+  leftBracket: {
+  rightBracket: }
+''');
+  }
+
   void test_class_macro() {
     var parseResult = parseStringWithErrors(r'''
 macro class A {}
@@ -419,6 +459,26 @@ ClassDeclaration
 ''');
   }
 
+  void test_class_sealed_mixin() {
+    var parseResult = parseStringWithErrors(r'''
+sealed mixin class A {}
+''');
+    parseResult.assertErrors([
+      error(ParserErrorCode.SEALED_MIXIN_CLASS, 0, 6),
+    ]);
+
+    var node = parseResult.findNode.classDeclaration('class A {}');
+    assertParsedNodeText(node, r'''
+ClassDeclaration
+  sealedKeyword: sealed
+  mixinKeyword: mixin
+  classKeyword: class
+  name: A
+  leftBracket: {
+  rightBracket: }
+''');
+  }
+
   void test_class_withClause_recordType() {
     var parseResult = parseStringWithErrors(r'''
 class C with A, (int, int), B {}

pkg/front_end/messages.yaml

@@ -6068,6 +6068,33 @@ CantUseClassAsMixin:
     lib.dart:
       class A {}
 
+FinalMixinClass:
+  index: 142
+  problemMessage: "A mixin class can't be declared 'final'."
+  correctionMessage: "Try removing the 'final' keyword."
+  analyzerCode: ParserErrorCode.FINAL_MIXIN_CLASS
+  experiments: class-modifiers
+  script:
+    - "final mixin class C {}"
+
+InterfaceMixinClass:
+  index: 143
+  problemMessage: "A mixin class can't be declared 'interface'."
+  correctionMessage: "Try removing the 'interface' keyword."
+  analyzerCode: ParserErrorCode.INTERFACE_MIXIN_CLASS
+  experiments: class-modifiers
+  script:
+    - "interface mixin class C {}"
+
+SealedMixinClass:
+  index: 144
+  problemMessage: "A mixin class can't be declared 'sealed'."
+  correctionMessage: "Try removing the 'sealed' keyword."
+  analyzerCode: ParserErrorCode.SEALED_MIXIN_CLASS
+  experiments: class-modifiers,sealed-class
+  script:
+    - "sealed mixin class C {}"
+
 BaseClassImplementedOutsideOfLibrary:
   problemMessage: "The class '#name' can't be implemented outside of its library because it's a base class."
   analyzerCode: BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY
@@ -6458,4 +6485,4 @@ PatternAssignmentNotLocalVariable:
     var global;
     method(x) {
       [global] = x;
-    }
+    }

pkg/front_end/testcases/class_modifiers/folder.options

@@ -1 +1 @@
---enable-experiment=class-modifiers
+--enable-experiment=class-modifiers,sealed-class

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart

@@ -0,0 +1,7 @@
+// 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.
+
+final mixin class A {}
+interface mixin class B {}
+sealed mixin class C {}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.strong.expect

@@ -0,0 +1,37 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    : super core::Object::•()
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.strong.transformed.expect

@@ -0,0 +1,37 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    : super core::Object::•()
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.textual_outline.expect

@@ -0,0 +1,6 @@
+final mixin
+class A {}
+interface mixin
+class B {}
+sealed mixin
+class C {}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.weak.expect

@@ -0,0 +1,37 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    : super core::Object::•()
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.weak.modular.expect

@@ -0,0 +1,37 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    : super core::Object::•()
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.weak.outline.expect

@@ -0,0 +1,34 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart.weak.transformed.expect

@@ -0,0 +1,37 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:5:1: Error: A mixin class can't be declared 'final'.
+// Try removing the 'final' keyword.
+// final mixin class A {}
+// ^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:6:1: Error: A mixin class can't be declared 'interface'.
+// Try removing the 'interface' keyword.
+// interface mixin class B {}
+// ^^^^^^^^^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/mixin_class_invalid_modifiers.dart:7:1: Error: A mixin class can't be declared 'sealed'.
+// Try removing the 'sealed' keyword.
+// sealed mixin class C {}
+// ^^^^^^
+//
+import self as self;
+import "dart:core" as core;
+
+final mixin class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+interface mixin class B extends core::Object {
+  synthetic constructor •() → self::B
+    : super core::Object::•()
+    ;
+}
+abstract sealed mixin class C extends core::Object {
+  synthetic constructor •() → self::C
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/textual_outline.status

@@ -26,6 +26,7 @@ class_modifiers/final/final_class_declaration: FormatterCrash
 class_modifiers/interface/interface_class_declaration: FormatterCrash
 class_modifiers/mixin/mixin_class_declaration: FormatterCrash
 class_modifiers/mixin/mixin_class_generative_constructor: FormatterCrash
+class_modifiers/mixin/mixin_class_invalid_modifiers: FormatterCrash
 class_modifiers/mixin/mixin_class_superclass_not_object: FormatterCrash
 const_functions/const_functions_const_ctor: FormatterCrash
 const_functions/const_functions_const_ctor_error: FormatterCrash

tests/language/class_modifiers/mixin/mixin_class_base_test.dart

@@ -0,0 +1,17 @@
+// 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
+
+// Allow base mixin classes.
+
+import 'package:expect/expect.dart';
+
+base mixin class BaseMixinClass {
+  int foo = 0;
+}
+
+main() {
+  Expect.equals(0, BaseMixinClass().foo);
+}

tests/language/class_modifiers/mixin/mixin_class_invalid_modifier_error_test.dart

@@ -0,0 +1,22 @@
+// 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,sealed-class
+
+// Error when applying any other modifier to mixin classes other than base.
+
+final mixin class A {}
+// [error column 1, length 5]
+// [analyzer] SYNTACTIC_ERROR.FINAL_MIXIN_CLASS
+// [cfe] A mixin class can't be declared 'final'.
+
+interface mixin class B {}
+// [error column 1, length 9]
+// [analyzer] SYNTACTIC_ERROR.INTERFACE_MIXIN_CLASS
+// [cfe] A mixin class can't be declared 'interface'.
+
+sealed mixin class C {}
+// [error column 1, length 6]
+// [analyzer] SYNTACTIC_ERROR.SEALED_MIXIN_CLASS
+// [cfe] A mixin class can't be declared 'sealed'.