Augment. Report augmentationOfMixinApplicationClass.

Change-Id: I2649f4eb96fc232727fafa16129799ef3f207e20
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494340
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

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

@@ -241,6 +241,8 @@ augmentation_modifier_missing:
   status: needsFix
 augmentation_of_different_declaration_kind:
   status: noFix
+augmentation_of_mixin_application_class:
+  status: needsEvaluation
 augmentation_type_parameter_bound:
   status: noFix
 augmentation_type_parameter_count:

pkg/analyzer/lib/src/diagnostic/diagnostic.g.dart

@@ -943,6 +943,19 @@ augmentationOfDifferentDeclarationKind = DiagnosticWithArguments(
   expectedTypes: [ExpectedType.string, ExpectedType.string],
 );
 
+/// No parameters.
+const DiagnosticWithoutArguments augmentationOfMixinApplicationClass =
+    DiagnosticWithoutArgumentsImpl(
+      name: 'augmentation_of_mixin_application_class',
+      problemMessage: "Mixin application classes can't be augmented.",
+      correctionMessage:
+          "Try removing the 'augment' keyword, or making the target a normal "
+          "class.",
+      type: DiagnosticType.COMPILE_TIME_ERROR,
+      uniqueName: 'augmentation_of_mixin_application_class',
+      expectedTypes: [],
+    );
+
 /// No parameters.
 const DiagnosticWithoutArguments augmentationTypeParameterBound =
     DiagnosticWithoutArgumentsImpl(

pkg/analyzer/lib/src/diagnostic/diagnostic_code_values.g.dart

@@ -73,6 +73,7 @@ const List<DiagnosticCode> diagnosticCodeValues = [
   diag.augmentationModifierExtra,
   diag.augmentationModifierMissing,
   diag.augmentationOfDifferentDeclarationKind,
+  diag.augmentationOfMixinApplicationClass,
   diag.augmentationTypeParameterBound,
   diag.augmentationTypeParameterCount,
   diag.augmentationTypeParameterName,

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -2018,33 +2018,38 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     if (augmentKeyword != null) {
       if (fragment.previousFragment == null) {
         var element = fragment.element;
-        if (element.previousFragmentOfDifferentKind case var previous?) {
-          diagnosticReporter.report(
-            diag.augmentationOfDifferentDeclarationKind
-                .withArguments(
-                  declarationKind: previous.element.kind.displayName,
-                  augmentationKind: element.kind.displayName,
-                )
-                .withContextMessages([
-                  if (previous case FragmentImpl(
-                    libraryFragment: var libraryFragment?,
-                    name: var name?,
-                    nameOffset: var nameOffset?,
-                  ))
-                    DiagnosticMessageImpl(
-                      filePath: libraryFragment.source.fullName,
-                      message: "The declaration being augmented.",
-                      offset: nameOffset,
-                      length: name.length,
-                      url: null,
+        var previousFragmentOfDifferentKind =
+            element.previousFragmentOfDifferentKind;
+        switch (previousFragmentOfDifferentKind) {
+          case ClassFragmentImpl(isMixinApplication: true):
+            diagnosticReporter.report(
+              diag.augmentationOfMixinApplicationClass
+                  .withContextMessages([
+                    ?previousFragmentOfDifferentKind.contextMessageAt(
+                      "The declaration being augmented.",
                     ),
-                ])
-                .at(augmentKeyword),
-          );
-        } else {
-          diagnosticReporter.report(
-            diag.augmentationWithoutDeclaration.at(augmentKeyword),
-          );
+                  ])
+                  .at(augmentKeyword),
+            );
+          case FragmentImpl previousFragment:
+            var previousElement = previousFragment.element;
+            diagnosticReporter.report(
+              diag.augmentationOfDifferentDeclarationKind
+                  .withArguments(
+                    declarationKind: previousElement.kind.displayName,
+                    augmentationKind: element.kind.displayName,
+                  )
+                  .withContextMessages([
+                    ?previousFragment.contextMessageAt(
+                      "The declaration being augmented.",
+                    ),
+                  ])
+                  .at(augmentKeyword),
+            );
+          case null:
+            diagnosticReporter.report(
+              diag.augmentationWithoutDeclaration.at(augmentKeyword),
+            );
         }
       }
     }

pkg/analyzer/lib/src/summary2/element_builder.dart

@@ -203,7 +203,9 @@ class ElementBuilder {
   ) {
     libraryFragment.addClass(fragment);
 
-    if (fragment.isAugmentation && lastFragment is ClassFragmentImpl) {
+    if (fragment.isAugmentation &&
+        lastFragment is ClassFragmentImpl &&
+        !lastFragment.isMixinApplication) {
       lastFragment.addFragment(fragment);
 
       _linkTypeParameters(

pkg/analyzer/lib/src/utilities/extensions/element.dart

@@ -8,6 +8,7 @@ import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart';
 import 'package:analyzer/src/dart/element/type.dart';
+import 'package:analyzer/src/diagnostic/diagnostic_message.dart';
 import 'package:meta/meta.dart';
 
 class MockLibraryImportElement implements Element {
@@ -260,6 +261,24 @@ extension FormalParameterElementImplExtension on FormalParameterElementImpl {
   }
 }
 
+extension FragmentImplExtension on FragmentImpl {
+  DiagnosticMessageImpl? contextMessageAt(String message) {
+    var libraryFragment = this.libraryFragment;
+    var nameOffset = this.nameOffset;
+    var name = this.name;
+    if (libraryFragment != null && nameOffset != null && name != null) {
+      return DiagnosticMessageImpl(
+        filePath: libraryFragment.source.fullName,
+        message: message,
+        offset: nameOffset,
+        length: name.length,
+        url: null,
+      );
+    }
+    return null;
+  }
+}
+
 extension GetterElementImplExtension on GetterElementImpl {
   PropertyAccessorFragmentImpl get asElement {
     return lastFragment;

pkg/analyzer/messages.yaml

@@ -1869,6 +1869,13 @@ CompileTimeErrorCode:
     problemMessage: The declaration being augmented doesn't exist.
     correctionMessage: Try changing the augmentation to match an existing declaration.
     hasPublishedDocs: false
+  augmentationOfMixinApplicationClass:
+    type: compileTimeError
+    parameters: none
+    experiment: augmentations
+    problemMessage: "Mixin application classes can't be augmented."
+    correctionMessage: Try removing the 'augment' keyword, or making the target a normal class.
+    hasPublishedDocs: false
   augmentedExpressionIsNotSetter:
     type: compileTimeError
     parameters: none

pkg/analyzer/test/src/diagnostics/augmentation_of_mixin_application_class_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2026, 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.
+
+import 'package:analyzer/src/diagnostic/diagnostic.dart' as diag;
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../dart/resolution/context_collection_resolution.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(AugmentationOfMixinApplicationClassTest);
+  });
+}
+
+@reflectiveTest
+class AugmentationOfMixinApplicationClassTest extends PubPackageResolutionTest {
+  test_class() async {
+    await assertErrorsInCode(
+      r'''
+class A {}
+mixin M {}
+class C = A with M;
+augment class C {}
+''',
+      [
+        error(
+          diag.augmentationOfMixinApplicationClass,
+          42,
+          7,
+          contextMessages: [message(testFile, 28, 1)],
+        ),
+      ],
+    );
+  }
+}

pkg/analyzer/test/src/diagnostics/test_all.dart

@@ -47,6 +47,8 @@ import 'augmentation_modifier_missing_test.dart'
     as augmentation_modifier_missing;
 import 'augmentation_of_different_declaration_kind_test.dart'
     as augmentation_of_different_declaration_kind;
+import 'augmentation_of_mixin_application_class_test.dart'
+    as augmentation_of_mixin_application_class;
 import 'augmentation_type_parameter_bound_test.dart'
     as augmentation_type_parameter_bound;
 import 'augmentation_type_parameter_count_test.dart'
@@ -984,6 +986,7 @@ main() {
     augmentation_modifier_extra.main();
     augmentation_modifier_missing.main();
     augmentation_of_different_declaration_kind.main();
+    augmentation_of_mixin_application_class.main();
     augmentation_type_parameter_bound.main();
     augmentation_type_parameter_count.main();
     augmentation_type_parameter_name.main();

pkg/analyzer/test/src/summary/elements/class_test.dart

@@ -26038,6 +26038,70 @@ library
 ''');
   }
 
+  test_augmentation_mixinApplication() async {
+    var library = await buildLibrary(r'''
+mixin M {}
+class C = Object with M;
+augment class C {}
+''');
+
+    checkElementText(library, r'''
+library
+  reference: <testLibrary>
+  fragments
+    #F0 <testLibraryFragment>
+      element: <testLibrary>
+      classes
+        #F1 isMixinApplication class C (nameOffset:17) (firstTokenOffset:11) (offset:17)
+          element: <testLibrary>::@class::C::@def::0
+          constructors
+            #F2 isConst isOriginMixinApplication new (nameOffset:<null>) (firstTokenOffset:<null>) (offset:17)
+              element: <testLibrary>::@class::C::@def::0::@constructor::new
+              typeName: C
+        #F3 isAugmentation class C (nameOffset:50) (firstTokenOffset:36) (offset:50)
+          element: <testLibrary>::@class::C::@def::1
+          constructors
+            #F4 isOriginImplicitDefault new (nameOffset:<null>) (firstTokenOffset:<null>) (offset:50)
+              element: <testLibrary>::@class::C::@def::1::@constructor::new
+              typeName: C
+      mixins
+        #F5 mixin M (nameOffset:6) (firstTokenOffset:0) (offset:6)
+          element: <testLibrary>::@mixin::M
+  classes
+    isMixinApplication isSimplyBounded class C
+      reference: <testLibrary>::@class::C::@def::0
+      firstFragment: #F1
+      supertype: Object
+      mixins
+        M
+      constructors
+        isConst isOriginMixinApplication new
+          reference: <testLibrary>::@class::C::@def::0::@constructor::new
+          firstFragment: #F2
+          constantInitializers
+            SuperConstructorInvocation
+              superKeyword: super @0
+              argumentList: ArgumentList
+                leftParenthesis: ( @0
+                rightParenthesis: ) @0
+              element: dart:core::@class::Object::@constructor::new
+    isSimplyBounded class C
+      reference: <testLibrary>::@class::C::@def::1
+      firstFragment: #F3
+      previousFragmentOfDifferentKind: #F1
+      constructors
+        isOriginImplicitDefault new
+          reference: <testLibrary>::@class::C::@def::1::@constructor::new
+          firstFragment: #F4
+  mixins
+    isSimplyBounded mixin M
+      reference: <testLibrary>::@mixin::M
+      firstFragment: #F5
+      superclassConstraints
+        Object
+''');
+  }
+
   test_augmentationTarget() async {
     newFile('$testPackageLibPath/a1.dart', r'''
 part of 'test.dart';