[cfe] Check overrides on operators

Change-Id: Ib250117553b2e22c6c71b78bf354be1162bc9981
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151235
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/fasta/builder/class_builder.dart

@@ -62,7 +62,7 @@ import '../loader.dart';
 
 import '../modifier.dart';
 
-import '../names.dart' show noSuchMethodName;
+import '../names.dart' show equalsName, noSuchMethodName;
 
 import '../problems.dart' show internalProblem, unhandled, unimplemented;
 
@@ -791,27 +791,30 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
           "Constructor in override check.", declaredMember.fileOffset, fileUri);
     }
     if (declaredMember is Procedure && interfaceMember is Procedure) {
-      if (declaredMember.kind == ProcedureKind.Method &&
-          interfaceMember.kind == ProcedureKind.Method) {
-        bool seenCovariant = checkMethodOverride(
-            types, declaredMember, interfaceMember, isInterfaceCheck);
-        if (seenCovariant) {
-          handleSeenCovariant(
-              types, declaredMember, interfaceMember, isSetter, callback);
-        }
-      }
-      if (declaredMember.kind == ProcedureKind.Getter &&
-          interfaceMember.kind == ProcedureKind.Getter) {
-        checkGetterOverride(
-            types, declaredMember, interfaceMember, isInterfaceCheck);
-      }
-      if (declaredMember.kind == ProcedureKind.Setter &&
-          interfaceMember.kind == ProcedureKind.Setter) {
-        bool seenCovariant = checkSetterOverride(
-            types, declaredMember, interfaceMember, isInterfaceCheck);
-        if (seenCovariant) {
-          handleSeenCovariant(
-              types, declaredMember, interfaceMember, isSetter, callback);
+      if (declaredMember.kind == interfaceMember.kind) {
+        if (declaredMember.kind == ProcedureKind.Method ||
+            declaredMember.kind == ProcedureKind.Operator) {
+          bool seenCovariant = checkMethodOverride(
+              types, declaredMember, interfaceMember, isInterfaceCheck);
+          if (seenCovariant) {
+            handleSeenCovariant(
+                types, declaredMember, interfaceMember, isSetter, callback);
+          }
+        } else if (declaredMember.kind == ProcedureKind.Getter) {
+          checkGetterOverride(
+              types, declaredMember, interfaceMember, isInterfaceCheck);
+        } else if (declaredMember.kind == ProcedureKind.Setter) {
+          bool seenCovariant = checkSetterOverride(
+              types, declaredMember, interfaceMember, isInterfaceCheck);
+          if (seenCovariant) {
+            handleSeenCovariant(
+                types, declaredMember, interfaceMember, isSetter, callback);
+          }
+        } else {
+          assert(
+              false,
+              "Unexpected procedure kind in override check: "
+              "${declaredMember.kind}");
         }
       }
     } else {
@@ -1093,8 +1096,9 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
   @override
   bool checkMethodOverride(Types types, Procedure declaredMember,
       Procedure interfaceMember, bool isInterfaceCheck) {
-    assert(declaredMember.kind == ProcedureKind.Method);
-    assert(interfaceMember.kind == ProcedureKind.Method);
+    assert(declaredMember.kind == interfaceMember.kind);
+    assert(declaredMember.kind == ProcedureKind.Method ||
+        declaredMember.kind == ProcedureKind.Operator);
     bool seenCovariant = false;
     FunctionNode declaredFunction = declaredMember.function;
     FunctionNode interfaceFunction = interfaceMember.function;
@@ -1167,6 +1171,17 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
           declaredFunction.positionalParameters[i];
       VariableDeclaration interfaceParameter =
           interfaceFunction.positionalParameters[i];
+      if (i == 0 &&
+          declaredMember.name == equalsName &&
+          declaredParameter.type ==
+              types.hierarchy.coreTypes.objectNonNullableRawType &&
+          interfaceParameter.type is DynamicType) {
+        // TODO(johnniwinther): Add check for opt-in overrides of operator ==.
+        // `operator ==` methods in opt-out classes have type
+        // `bool Function(dynamic)`.
+        continue;
+      }
+
       _checkTypes(
           types,
           interfaceSubstitution,
@@ -1174,7 +1189,7 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
           declaredMember,
           interfaceMember,
           declaredParameter.type,
-          interfaceFunction.positionalParameters[i].type,
+          interfaceParameter.type,
           declaredParameter.isCovariant || interfaceParameter.isCovariant,
           declaredParameter,
           isInterfaceCheck);
@@ -1340,6 +1355,10 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
   void reportInvalidOverride(bool isInterfaceCheck, Member declaredMember,
       Message message, int fileOffset, int length,
       {List<LocatedMessage> context}) {
+    if (shouldOverrideProblemBeOverlooked(this)) {
+      return;
+    }
+
     if (declaredMember.enclosingClass == cls) {
       // Ordinary override
       library.addProblem(message, fileOffset, length, declaredMember.fileUri,
@@ -1769,3 +1788,17 @@ class ConstructorRedirection {
 
   ConstructorRedirection(this.target) : cycleReported = false;
 }
+
+/// Returns `true` if override problems should be overlooked.
+///
+/// This is needed for the current encoding of some JavaScript implementation
+/// classes that are not valid Dart. For instance `JSInt` in
+/// 'dart:_interceptors' that implements both `int` and `double`, and `JsArray`
+/// in `dart:js` that implement both `ListMixin` and `JsObject`.
+bool shouldOverrideProblemBeOverlooked(ClassBuilder classBuilder) {
+  String uri = '${classBuilder.library.importUri}';
+  return uri == 'dart:js' &&
+          classBuilder.fileUri.pathSegments.last == 'js.dart' ||
+      uri == 'dart:_interceptors' &&
+          classBuilder.fileUri.pathSegments.last == 'js_number.dart';
+}

pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart

@@ -3038,14 +3038,7 @@ class InterfaceConflict extends DelayedMember {
           debug?.log("Combined Member Signature: "
               "${bestSoFar.fullName} !<: ${candidate.fullName}");
 
-          String uri = '${classBuilder.library.importUri}';
-          if (uri == 'dart:js' &&
-                  classBuilder.fileUri.pathSegments.last == 'js.dart' ||
-              uri == 'dart:_interceptors' &&
-                  classBuilder.fileUri.pathSegments.last == 'js_number.dart') {
-            // TODO(johnniwinther): Fix the dart2js libraries and remove the
-            // above URIs.
-          } else {
+          if (!shouldOverrideProblemBeOverlooked(classBuilder)) {
             bestSoFar = null;
             bestTypeSoFar = null;
             mutualSubtypes = null;

pkg/front_end/test/spell_checking_list_code.txt

@@ -751,6 +751,7 @@ os
 outputting
 overlap
 overloader
+overlooked
 overshadowed
 overwrite
 overwriting

pkg/front_end/testcases/general/abstract_operator_override.dart

@@ -0,0 +1,24 @@
+// Copyright (c) 2020, 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.
+
+class A {
+  A operator +(B b) => new A();
+  A operator -() => new A();
+  A operator [](B b) => new A();
+  void operator []=(B b1, B b2) {}
+}
+
+class B extends A {
+  A operator +(A a);
+  B operator -();
+  A operator [](A a);
+  void operator []=(A a, B b);
+}
+
+class C extends A {
+  B operator [](B b);
+  void operator []=(B b, A a);
+}
+
+main() {}

pkg/front_end/testcases/general/abstract_operator_override.dart.outline.expect

@@ -0,0 +1,123 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '+' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:6:18: Context: The parameter 'b' of the method 'A.+' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.+'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator +(B b) => new A();
+//                  ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:13:14: Context: This is the overridden method ('+').
+//   A operator +(A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of 'unary-' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:7:14: Context: The return type of the method 'A.unary-' is 'A', which does not match the return type, 'B', of the overridden method, 'B.unary-'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator -() => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:14:14: Context: This is the overridden method ('unary-').
+//   B operator -();
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:19: Context: The parameter 'b' of the method 'A.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator [](B b) => new A();
+//                   ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:15:14: Context: This is the overridden method ('[]').
+//   A operator [](A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]=' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:23: Context: The parameter 'b1' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:16:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a, B b);
+//                 ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:14: Context: The return type of the method 'A.[]' is 'A', which does not match the return type, 'B', of the overridden method, 'C.[]'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator [](B b) => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:20:14: Context: This is the overridden method ('[]').
+//   B operator [](B b);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]=' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:29: Context: The parameter 'b2' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'C.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                             ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:21:17: Context: This is the overridden method ('[]=').
+//   void operator []=(B b, A a);
+//                 ^
+//
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A*
+    ;
+  operator +(self::B* b) → self::A*
+    ;
+  operator unary-() → self::A*
+    ;
+  operator [](self::B* b) → self::A*
+    ;
+  operator []=(self::B* b1, self::B* b2) → void
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B*
+    ;
+  abstract operator +(self::A* a) → self::A*;
+  abstract operator unary-() → self::B*;
+  abstract operator [](self::A* a) → self::A*;
+  abstract operator []=(self::A* a, self::B* b) → void;
+}
+class C extends self::A {
+  synthetic constructor •() → self::C*
+    ;
+  abstract operator [](self::B* b) → self::B*;
+  abstract operator []=(self::B* b, self::A* a) → void;
+}
+static method main() → dynamic
+  ;

pkg/front_end/testcases/general/abstract_operator_override.dart.strong.expect

@@ -0,0 +1,124 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '+' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:6:18: Context: The parameter 'b' of the method 'A.+' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.+'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator +(B b) => new A();
+//                  ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:13:14: Context: This is the overridden method ('+').
+//   A operator +(A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of 'unary-' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:7:14: Context: The return type of the method 'A.unary-' is 'A', which does not match the return type, 'B', of the overridden method, 'B.unary-'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator -() => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:14:14: Context: This is the overridden method ('unary-').
+//   B operator -();
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:19: Context: The parameter 'b' of the method 'A.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator [](B b) => new A();
+//                   ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:15:14: Context: This is the overridden method ('[]').
+//   A operator [](A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]=' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:23: Context: The parameter 'b1' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:16:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a, B b);
+//                 ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:14: Context: The return type of the method 'A.[]' is 'A', which does not match the return type, 'B', of the overridden method, 'C.[]'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator [](B b) => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:20:14: Context: This is the overridden method ('[]').
+//   B operator [](B b);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]=' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:29: Context: The parameter 'b2' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'C.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                             ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:21:17: Context: This is the overridden method ('[]=').
+//   void operator []=(B b, A a);
+//                 ^
+//
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A*
+    : super core::Object::•()
+    ;
+  operator +(self::B* b) → self::A*
+    return new self::A::•();
+  operator unary-() → self::A*
+    return new self::A::•();
+  operator [](self::B* b) → self::A*
+    return new self::A::•();
+  operator []=(self::B* b1, self::B* b2) → void {}
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B*
+    : super self::A::•()
+    ;
+  abstract operator +(self::A* a) → self::A*;
+  abstract operator unary-() → self::B*;
+  abstract operator [](self::A* a) → self::A*;
+  abstract operator []=(self::A* a, self::B* b) → void;
+}
+class C extends self::A {
+  synthetic constructor •() → self::C*
+    : super self::A::•()
+    ;
+  abstract operator [](self::B* b) → self::B*;
+  abstract operator []=(self::B* b, self::A* a) → void;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/general/abstract_operator_override.dart.strong.transformed.expect

@@ -0,0 +1,124 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '+' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:6:18: Context: The parameter 'b' of the method 'A.+' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.+'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator +(B b) => new A();
+//                  ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:13:14: Context: This is the overridden method ('+').
+//   A operator +(A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of 'unary-' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:7:14: Context: The return type of the method 'A.unary-' is 'A', which does not match the return type, 'B', of the overridden method, 'B.unary-'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator -() => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:14:14: Context: This is the overridden method ('unary-').
+//   B operator -();
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:19: Context: The parameter 'b' of the method 'A.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator [](B b) => new A();
+//                   ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:15:14: Context: This is the overridden method ('[]').
+//   A operator [](A a);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:12:7: Error: The implementation of '[]=' in the non-abstract class 'B' does not conform to its interface.
+// class B extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:23: Context: The parameter 'b1' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'B.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:16:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a, B b);
+//                 ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:8:14: Context: The return type of the method 'A.[]' is 'A', which does not match the return type, 'B', of the overridden method, 'C.[]'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator [](B b) => new A();
+//              ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:20:14: Context: This is the overridden method ('[]').
+//   B operator [](B b);
+//              ^
+//
+// pkg/front_end/testcases/general/abstract_operator_override.dart:19:7: Error: The implementation of '[]=' in the non-abstract class 'C' does not conform to its interface.
+// class C extends A {
+//       ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:9:29: Context: The parameter 'b2' of the method 'A.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'C.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/abstract_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b1, B b2) {}
+//                             ^
+// pkg/front_end/testcases/general/abstract_operator_override.dart:21:17: Context: This is the overridden method ('[]=').
+//   void operator []=(B b, A a);
+//                 ^
+//
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A*
+    : super core::Object::•()
+    ;
+  operator +(self::B* b) → self::A*
+    return new self::A::•();
+  operator unary-() → self::A*
+    return new self::A::•();
+  operator [](self::B* b) → self::A*
+    return new self::A::•();
+  operator []=(self::B* b1, self::B* b2) → void {}
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B*
+    : super self::A::•()
+    ;
+  abstract operator +(self::A* a) → self::A*;
+  abstract operator unary-() → self::B*;
+  abstract operator [](self::A* a) → self::A*;
+  abstract operator []=(self::A* a, self::B* b) → void;
+}
+class C extends self::A {
+  synthetic constructor •() → self::C*
+    : super self::A::•()
+    ;
+  abstract operator [](self::B* b) → self::B*;
+  abstract operator []=(self::B* b, self::A* a) → void;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/general/abstract_operator_override.dart.textual_outline.expect

@@ -0,0 +1,20 @@
+class A {
+  A operator +(B b) => new A();
+  A operator -() => new A();
+  A operator [](B b) => new A();
+  void operator []=(B b1, B b2) {}
+}
+
+class B extends A {
+  A operator +(A a);
+  B operator -();
+  A operator [](A a);
+  void operator []=(A a, B b);
+}
+
+class C extends A {
+  B operator [](B b);
+  void operator []=(B b, A a);
+}
+
+main() {}

pkg/front_end/testcases/general/abstract_operator_override.dart.textual_outline_modelled.expect

@@ -0,0 +1,20 @@
+class A {
+  A operator +(B b) => new A();
+  A operator -() => new A();
+  A operator [](B b) => new A();
+  void operator []=(B b1, B b2) {}
+}
+
+class B extends A {
+  A operator +(A a);
+  A operator [](A a);
+  B operator -();
+  void operator []=(A a, B b);
+}
+
+class C extends A {
+  B operator [](B b);
+  void operator []=(B b, A a);
+}
+
+main() {}

pkg/front_end/testcases/general/invalid_operator.dart.outline.expect

@@ -560,6 +560,42 @@ library;
 //   operator ~<T>() => true;
 //              ^
 //
+// pkg/front_end/testcases/general/invalid_operator.dart:6:12: Error: The method 'Operators1.==' has fewer positional arguments than those of overridden method 'Object.=='.
+//   operator ==() => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:27:12: Error: The method 'Operators2.==' has more required arguments than those of overridden method 'Object.=='.
+//   operator ==(a, b) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:71:12: Error: The method 'Operators4.==' has fewer positional arguments than those of overridden method 'Object.=='.
+//   operator ==({a}) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:137:12: Error: Declared type variables of 'Operators7.==' doesn't match those on overridden method 'Object.=='.
+//   operator ==<T>(a) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:137:12: Error: The return type of the method 'Operators7.==' is 'dynamic', which does not match the return type, 'bool', of the overridden method, 'Object.=='.
+// Change to a subtype of 'bool'.
+//   operator ==<T>(a) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/general/invalid_operator.dart.strong.expect

@@ -560,6 +560,42 @@ library;
 //   operator ~<T>() => true;
 //              ^
 //
+// pkg/front_end/testcases/general/invalid_operator.dart:6:12: Error: The method 'Operators1.==' has fewer positional arguments than those of overridden method 'Object.=='.
+//   operator ==() => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:27:12: Error: The method 'Operators2.==' has more required arguments than those of overridden method 'Object.=='.
+//   operator ==(a, b) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:71:12: Error: The method 'Operators4.==' has fewer positional arguments than those of overridden method 'Object.=='.
+//   operator ==({a}) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:137:12: Error: Declared type variables of 'Operators7.==' doesn't match those on overridden method 'Object.=='.
+//   operator ==<T>(a) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator.dart:137:12: Error: The return type of the method 'Operators7.==' is 'dynamic', which does not match the return type, 'bool', of the overridden method, 'Object.=='.
+// Change to a subtype of 'bool'.
+//   operator ==<T>(a) => true;
+//            ^
+// sdk/lib/_internal/vm/lib/object_patch.dart:18:17: Context: This is the overridden method ('==').
+//   bool operator ==(Object other) native "Object_equals";
+//                 ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/general/invalid_operator_override.dart

@@ -0,0 +1,24 @@
+// Copyright (c) 2020, 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.
+
+class A {
+  A operator +(A a) => a;
+  B operator -() => new B();
+  B operator [](A a) => new B();
+  void operator []=(A a1, A a2) {}
+}
+
+class B extends A {
+  A operator +(B b) => b;
+  A operator -() => this;
+  B operator [](B b) => b;
+  void operator []=(B b, A a) {}
+}
+
+class C extends A {
+  A operator [](B b) => b;
+  void operator []=(A a, B b) {}
+}
+
+main() {}

pkg/front_end/testcases/general/invalid_operator_override.dart.outline.expect

@@ -0,0 +1,121 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:13:18: Error: The parameter 'b' of the method 'B.+' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.+'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator +(B b) => b;
+//                  ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:6:14: Context: This is the overridden method ('+').
+//   A operator +(A a) => a;
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:14:14: Error: The return type of the method 'B.unary-' is 'A', which does not match the return type, 'B', of the overridden method, 'A.unary-'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator -() => this;
+//              ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:7:14: Context: This is the overridden method ('unary-').
+//   B operator -() => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:15:19: Error: The parameter 'b' of the method 'B.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   B operator [](B b) => b;
+//                   ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:16:23: Error: The parameter 'b' of the method 'B.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b, A a) {}
+//                       ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:9:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a1, A a2) {}
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:20:14: Error: The return type of the method 'C.[]' is 'A', which does not match the return type, 'B', of the overridden method, 'A.[]'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator [](B b) => b;
+//              ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:20:19: Error: The parameter 'b' of the method 'C.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator [](B b) => b;
+//                   ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:21:28: Error: The parameter 'b' of the method 'C.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(A a, B b) {}
+//                            ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:9:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a1, A a2) {}
+//                 ^
+//
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A*
+    ;
+  operator +(self::A* a) → self::A*
+    ;
+  operator unary-() → self::B*
+    ;
+  operator [](self::A* a) → self::B*
+    ;
+  operator []=(self::A* a1, self::A* a2) → void
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B*
+    ;
+  operator +(self::B* b) → self::A*
+    ;
+  operator unary-() → self::A*
+    ;
+  operator [](self::B* b) → self::B*
+    ;
+  operator []=(self::B* b, self::A* a) → void
+    ;
+}
+class C extends self::A {
+  synthetic constructor •() → self::C*
+    ;
+  operator [](self::B* b) → self::A*
+    ;
+  operator []=(self::A* a, self::B* b) → void
+    ;
+}
+static method main() → dynamic
+  ;

pkg/front_end/testcases/general/invalid_operator_override.dart.strong.expect

@@ -0,0 +1,120 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:13:18: Error: The parameter 'b' of the method 'B.+' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.+'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator +(B b) => b;
+//                  ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:6:14: Context: This is the overridden method ('+').
+//   A operator +(A a) => a;
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:14:14: Error: The return type of the method 'B.unary-' is 'A', which does not match the return type, 'B', of the overridden method, 'A.unary-'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator -() => this;
+//              ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:7:14: Context: This is the overridden method ('unary-').
+//   B operator -() => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:15:19: Error: The parameter 'b' of the method 'B.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   B operator [](B b) => b;
+//                   ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:16:23: Error: The parameter 'b' of the method 'B.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(B b, A a) {}
+//                       ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:9:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a1, A a2) {}
+//                 ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:20:14: Error: The return type of the method 'C.[]' is 'A', which does not match the return type, 'B', of the overridden method, 'A.[]'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a subtype of 'B'.
+//   A operator [](B b) => b;
+//              ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:20:19: Error: The parameter 'b' of the method 'C.[]' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]'.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   A operator [](B b) => b;
+//                   ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:8:14: Context: This is the overridden method ('[]').
+//   B operator [](A a) => new B();
+//              ^
+//
+// pkg/front_end/testcases/general/invalid_operator_override.dart:21:28: Error: The parameter 'b' of the method 'C.[]=' has type 'B', which does not match the corresponding type, 'A', in the overridden method, 'A.[]='.
+//  - 'B' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+//  - 'A' is from 'pkg/front_end/testcases/general/invalid_operator_override.dart'.
+// Change to a supertype of 'A', or, for a covariant parameter, a subtype.
+//   void operator []=(A a, B b) {}
+//                            ^
+// pkg/front_end/testcases/general/invalid_operator_override.dart:9:17: Context: This is the overridden method ('[]=').
+//   void operator []=(A a1, A a2) {}
+//                 ^
+//
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A*
+    : super core::Object::•()
+    ;
+  operator +(self::A* a) → self::A*
+    return a;
+  operator unary-() → self::B*
+    return new self::B::•();
+  operator [](self::A* a) → self::B*
+    return new self::B::•();
+  operator []=(self::A* a1, self::A* a2) → void {}
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B*
+    : super self::A::•()
+    ;
+  operator +(self::B* b) → self::A*
+    return b;
+  operator unary-() → self::A*
+    return this;
+  operator [](self::B* b) → self::B*
+    return b;
+  operator []=(self::B* b, self::A* a) → void {}
+}
+class C extends self::A {
+  synthetic constructor •() → self::C*
+    : super self::A::•()
+    ;
+  operator [](self::B* b) → self::A*
+    return b;
+  operator []=(self::A* a, self::B* b) → void {}
+}
+static method main() → dynamic {}

pkg/front_end/testcases/general/invalid_operator_override.dart.textual_outline.expect

@@ -0,0 +1,20 @@
+class A {
+  A operator +(A a) => a;
+  B operator -() => new B();
+  B operator [](A a) => new B();
+  void operator []=(A a1, A a2) {}
+}
+
+class B extends A {
+  A operator +(B b) => b;
+  A operator -() => this;
+  B operator [](B b) => b;
+  void operator []=(B b, A a) {}
+}
+
+class C extends A {
+  A operator [](B b) => b;
+  void operator []=(A a, B b) {}
+}
+
+main() {}

pkg/front_end/testcases/general/invalid_operator_override.dart.textual_outline_modelled.expect

@@ -0,0 +1,20 @@
+class A {
+  A operator +(A a) => a;
+  B operator -() => new B();
+  B operator [](A a) => new B();
+  void operator []=(A a1, A a2) {}
+}
+
+class B extends A {
+  A operator +(B b) => b;
+  A operator -() => this;
+  B operator [](B b) => b;
+  void operator []=(B b, A a) {}
+}
+
+class C extends A {
+  A operator [](B b) => b;
+  void operator []=(A a, B b) {}
+}
+
+main() {}

pkg/front_end/testcases/nnbd/issue42199.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2020, 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.
+
+class A {
+  operator ==(dynamic other);
+}
+
+main() {}

pkg/front_end/testcases/nnbd/issue42199.dart.outline.expect

@@ -0,0 +1,11 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+  abstract operator ==(dynamic other) → core::bool;
+}
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/issue42199.dart.strong.expect

@@ -0,0 +1,11 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator ==(dynamic other) → core::bool;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue42199.dart.strong.transformed.expect

@@ -0,0 +1,11 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator ==(dynamic other) → core::bool;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue42199.dart.textual_outline.expect

@@ -0,0 +1,5 @@
+class A {
+  operator ==(dynamic other);
+}
+
+main() {}

pkg/front_end/testcases/nnbd/issue42199.dart.textual_outline_modelled.expect

@@ -0,0 +1,5 @@
+class A {
+  operator ==(dynamic other);
+}
+
+main() {}

pkg/front_end/testcases/nnbd/issue42199.dart.weak.expect

@@ -0,0 +1,11 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator ==(dynamic other) → core::bool;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue42199.dart.weak.transformed.expect

@@ -0,0 +1,11 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator ==(dynamic other) → core::bool;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/outline.status

@@ -17,6 +17,7 @@ general/bug30695: TypeCheckError
 general/covariant_field: TypeCheckError
 general/infer_field_from_multiple: TypeCheckError
 general/invalid_operator: TypeCheckError
+general/invalid_operator_override: TypeCheckError
 general/issue41210a: TypeCheckError
 general/issue41210b/issue41210: TypeCheckError
 general/mixin_application_override: TypeCheckError

pkg/front_end/testcases/strong.status

@@ -65,6 +65,7 @@ general/external_import: RuntimeError # The native extension to import doesn't e
 general/incomplete_field_formal_parameter: RuntimeError
 general/infer_field_from_multiple: TypeCheckError
 general/invalid_operator: TypeCheckError
+general/invalid_operator_override: TypeCheckError
 general/invocations: RuntimeError
 general/issue37776: RuntimeError
 general/issue38938: RuntimeError # no main and compile time errors.

pkg/front_end/testcases/text_serialization.status

@@ -65,6 +65,7 @@ general/external_import: RuntimeError
 general/incomplete_field_formal_parameter: RuntimeError
 general/infer_field_from_multiple: TypeCheckError
 general/invalid_operator: TypeCheckError
+general/invalid_operator_override: TypeCheckError
 general/invocations: RuntimeError
 general/issue37776: RuntimeError
 general/issue38938: RuntimeError

runtime/vm/object_test.cc

@@ -4325,7 +4325,7 @@ TEST_CASE(InstanceEquality) {
   // Test that Instance::OperatorEquals can call a user-defined operator==.
   const char* kScript =
       "class A {\n"
-      "  bool operator==(A other) { return true; }\n"
+      "  bool operator==(covariant A other) { return true; }\n"
       "}\n"
       "main() {\n"
       "  A a = new A();\n"

samples-dev/swarm/swarm_ui_lib/base/Size.dart

@@ -13,7 +13,7 @@ class Size {
 
   Size(num this.width, num this.height) {}
 
-  bool operator ==(Size other) {
+  bool operator ==(covariant Size other) {
     return other != null && width == other.width && height == other.height;
   }
 

samples-dev/swarm/swarm_ui_lib/touch/Geometry.dart

@@ -36,7 +36,7 @@ class Coordinate {
     return Math.sqrt(dx * dx + dy * dy);
   }
 
-  bool operator ==(Coordinate other) {
+  bool operator ==(covariant Coordinate other) {
     return other != null && x == other.x && y == other.y;
   }
 
@@ -73,7 +73,7 @@ class Interval {
     return end - start;
   }
 
-  bool operator ==(Interval other) {
+  bool operator ==(covariant Interval other) {
     return other != null && other.start == start && other.end == end;
   }
 

tests/language/operator/invalid_operators_test.dart

@@ -9,6 +9,7 @@ class Operators1 {
   // [cfe] Operator '==' should have exactly one parameter.
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
+  // [cfe] The method 'Operators1.==' has fewer positional arguments than those of overridden method 'Object.=='.
   operator <() => true;
   //       ^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
@@ -86,6 +87,7 @@ class Operators2 {
   // [cfe] Operator '==' should have exactly one parameter.
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
+  // [cfe] The method 'Operators2.==' has more required arguments than those of overridden method 'Object.=='.
   operator <(a, b) => true;
   //       ^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
@@ -249,6 +251,7 @@ class Operators4 {
   operator ==({a}) => true;
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.INVALID_OVERRIDE
+  // [cfe] The method 'Operators4.==' has fewer positional arguments than those of overridden method 'Object.=='.
   //           ^
   // [analyzer] COMPILE_TIME_ERROR.OPTIONAL_PARAMETER_IN_OPERATOR
   // [cfe] An operator can't have optional parameters.
@@ -569,6 +572,9 @@ class Operators7 {
   operator ==<T>(a) => true;
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.INVALID_OVERRIDE
+  // [cfe] Declared type variables of 'Operators7.==' doesn't match those on overridden method 'Object.=='.
+  //       ^
+  // [cfe] The return type of the method 'Operators7.==' is 'dynamic', which does not match the return type, 'bool', of the overridden method, 'Object.=='.
   //         ^^^
   // [analyzer] SYNTACTIC_ERROR.TYPE_PARAMETERS_ON_OPERATOR
   //          ^

tests/language_2/operator/invalid_operators_test.dart

@@ -9,6 +9,7 @@ class Operators1 {
   // [cfe] Operator '==' should have exactly one parameter.
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
+  // [cfe] The method 'Operators1.==' has fewer positional arguments than those of overridden method 'Object.=='.
   operator <() => true;
   //       ^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
@@ -86,6 +87,7 @@ class Operators2 {
   // [cfe] Operator '==' should have exactly one parameter.
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
+  // [cfe] The method 'Operators2.==' has more required arguments than those of overridden method 'Object.=='.
   operator <(a, b) => true;
   //       ^
   // [analyzer] COMPILE_TIME_ERROR.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR
@@ -248,6 +250,7 @@ class Operators4 {
   operator ==({a}) => true;
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.INVALID_OVERRIDE
+  // [cfe] The method 'Operators4.==' has fewer positional arguments than those of overridden method 'Object.=='.
   //           ^
   // [analyzer] COMPILE_TIME_ERROR.OPTIONAL_PARAMETER_IN_OPERATOR
   // [cfe] An operator can't have optional parameters.
@@ -568,6 +571,9 @@ class Operators7 {
   operator ==<T>(a) => true;
   //       ^^
   // [analyzer] COMPILE_TIME_ERROR.INVALID_OVERRIDE
+  // [cfe] Declared type variables of 'Operators7.==' doesn't match those on overridden method 'Object.=='.
+  //       ^
+  // [cfe] The return type of the method 'Operators7.==' is 'dynamic', which does not match the return type, 'bool', of the overridden method, 'Object.=='.
   //         ^^^
   // [analyzer] SYNTACTIC_ERROR.TYPE_PARAMETERS_ON_OPERATOR
   //          ^