Add strong mode error for mixins defining conflicting private names (issue 28809)

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

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 1fa5eae09db4805cfbc74551a068dbc160ea4184..2c9ef54c5f7ee17ab91159945af24c7247291277 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -486,6 +486,9 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
           _checkImplementsSuperClass(node);
           _checkImplementsFunctionWithoutCall(node);
           _checkForMixinHasNoConstructors(node);
+          if (_options.strongMode) {
+            _checkForMixinWithConflictingPrivateMember(node);
+          }
         }
       }
       visitClassDeclarationIncrementally(node);
@@ -4899,6 +4902,80 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
   }
 
   /**
+   * Check for the declaration of a mixin from a library other than the current
+   * library that defines a private member that conflicts with a private name
+   * from the same library but from a superclass or a different mixin.
+   */
+  void _checkForMixinWithConflictingPrivateMember(ClassDeclaration node) {
+    WithClause withClause = node.withClause;
+    if (withClause == null) {
+      return;
+    }
+    DartType declaredSupertype = node.extendsClause?.superclass?.type;
+    if (declaredSupertype is! InterfaceType) {
+      return;
+    }
+    InterfaceType superclass = declaredSupertype;
+    Map<LibraryElement, Map<String, String>> mixedInNames =
+        <LibraryElement, Map<String, String>>{};
+
+    /**
+     * Report an error and return `true` if the given [name] is a private name
+     * (which is defined in the given [library]) and it conflicts with another
+     * definition of that name inherited from the superclass.
+     */
+    bool isConflictingName(
+        String name, LibraryElement library, TypeName typeName) {
+      if (Identifier.isPrivateName(name)) {
+        Map<String, String> names =
+            mixedInNames.putIfAbsent(library, () => <String, String>{});
+        if (names.containsKey(name)) {
+          _errorReporter.reportErrorForNode(
+              CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION,
+              typeName,
+              [name, typeName.name.name, names[name]]);
+          return true;
+        }
+        names[name] = typeName.name.name;
+        ExecutableElement inheritedMember =
+            superclass.lookUpMethod(name, library) ??
+                superclass.lookUpGetter(name, library) ??
+                superclass.lookUpSetter(name, library);
+        if (inheritedMember != null) {
+          _errorReporter.reportErrorForNode(
+              CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION,
+              typeName, [
+            name,
+            typeName.name.name,
+            inheritedMember.enclosingElement.name
+          ]);
+          return true;
+        }
+      }
+      return false;
+    }
+
+    for (TypeName mixinType in withClause.mixinTypes) {
+      DartType type = mixinType.type;
+      if (type is InterfaceType) {
+        LibraryElement library = type.element.library;
+        if (library != _currentLibrary) {
+          for (PropertyAccessorElement accessor in type.accessors) {
+            if (isConflictingName(accessor.name, library, mixinType)) {
+              return;
+            }
+          }
+          for (MethodElement method in type.methods) {
+            if (isConflictingName(method.name, library, mixinType)) {
+              return;
+            }
+          }
+        }
+      }
+    }
+  }
+
+  /**
    * Verify that the given [constructor] has at most one 'super' initializer.
    *
    * See [CompileTimeErrorCode.MULTIPLE_SUPER_INITIALIZERS].