[cfe] Fix bad supertype substitution in mixin check.

Closes #42792

Change-Id: I21757f0f8417ad677d075e8417e390e1fa532a2b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155444
Reviewed-by: Dmitry Stefantsov <[email protected]>

Author: johnniwinther

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

@@ -1419,15 +1419,15 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
     InterfaceType supertype = cls.supertype.asInterfaceType;
     Substitution substitution = Substitution.fromSupertype(cls.mixedInType);
     for (Supertype constraint in cls.mixedInClass.superclassConstraints()) {
-      InterfaceType interface =
+      InterfaceType requiredInterface =
           substitution.substituteSupertype(constraint).asInterfaceType;
-      if (hierarchy.getTypeAsInstanceOf(
-              supertype, interface.classNode, library.library, coreTypes) !=
-          interface) {
+      InterfaceType implementedInterface = hierarchy.getTypeAsInstanceOf(
+          supertype, requiredInterface.classNode, library.library, coreTypes);
+      if (implementedInterface != requiredInterface) {
         library.addProblem(
             templateMixinApplicationIncompatibleSupertype.withArguments(
                 supertype,
-                interface,
+                requiredInterface,
                 cls.mixedInType.asInterfaceType,
                 library.isNonNullableByDefault),
             cls.fileOffset,

pkg/front_end/testcases/nnbd_mixed/issue42792.dart

@@ -0,0 +1,11 @@
+// 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.
+
+import 'issue42792_lib.dart';
+
+mixin M<T> on A<T> {}
+
+class B extends A<int> with M {}
+
+void main() {}

pkg/front_end/testcases/nnbd_mixed/issue42792.dart.textual_outline.expect

@@ -0,0 +1,7 @@
+import 'issue42792_lib.dart';
+
+mixin M<T> on A<T> {}
+
+class B extends A<int> with M {}
+
+void main() {}

pkg/front_end/testcases/nnbd_mixed/issue42792.dart.textual_outline_modelled.expect

@@ -0,0 +1,6 @@
+import 'issue42792_lib.dart';
+
+class B extends A<int> with M {}
+
+mixin M<T> on A<T> {}
+void main() {}

pkg/front_end/testcases/nnbd_mixed/issue42792.dart.weak.expect

@@ -0,0 +1,40 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "issue42792_lib.dart" as iss;
+
+import "org-dartlang-testcase:///issue42792_lib.dart";
+
+abstract class M<T extends core::Object? = dynamic> extends iss::A<self::M::T%> /*isMixinDeclaration*/  {
+}
+abstract class _B&A&M = iss::A<core::int> with self::M<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_B&A&M
+    : super iss::A::•()
+    ;
+}
+class B extends self::_B&A&M {
+  synthetic constructor •() → self::B
+    : super self::_B&A&M::•()
+    ;
+}
+static method main() → void {}
+
+library;
+import self as iss;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → iss::A<iss::A::T*>*
+    : super core::Object::•()
+    ;
+  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*;
+}

pkg/front_end/testcases/nnbd_mixed/issue42792.dart.weak.transformed.expect

@@ -0,0 +1,40 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "issue42792_lib.dart" as iss;
+
+import "org-dartlang-testcase:///issue42792_lib.dart";
+
+abstract class M<T extends core::Object? = dynamic> extends iss::A<self::M::T%> /*isMixinDeclaration*/  {
+}
+abstract class _B&A&M extends iss::A<core::int> implements self::M<core::int> /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_B&A&M
+    : super iss::A::•()
+    ;
+}
+class B extends self::_B&A&M {
+  synthetic constructor •() → self::B
+    : super self::_B&A&M::•()
+    ;
+}
+static method main() → void {}
+
+library;
+import self as iss;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → iss::A<iss::A::T*>*
+    : super core::Object::•()
+    ;
+  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*;
+}

pkg/front_end/testcases/nnbd_mixed/issue42792_lib.dart

@@ -0,0 +1,7 @@
+// 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.
+
+// @dart = 2.8
+
+abstract class A<T> {}

pkg/kernel/lib/class_hierarchy.dart

@@ -715,6 +715,19 @@ class ClosedWorldClassHierarchy implements ClassHierarchy {
   @override
   List<DartType> getTypeArgumentsAsInstanceOf(
       InterfaceType type, Class superclass) {
+    if (type.classNode == superclass) {
+      // TODO(johnniwinther): This is necessary because [getClassAsInstanceOf]
+      // returns a [Supertype] whose type arguments are type parameter types
+      // whose nullability is set to the default nullability of the
+      // enclosing library. If for instance [type] is `A<int!>` but `A` is
+      // declared in an opt-out library, the substitution below will combine
+      // nullabilities of the type arguments in [type] with the type parameters
+      // and thus give the result `A<int*>`. See issue #42792.
+      // For now we bypass the substitution but long term we need to ensure
+      // that [getClassAsInstanceOf] doesn't cause similar problems in other
+      // situations.
+      return type.typeArguments;
+    }
     Supertype castedType = getClassAsInstanceOf(type.classNode, superclass);
     if (castedType == null) return null;
     if (superclass.typeParameters.isEmpty) return const <DartType>[];