[cfe] Handle opt-out Null override of opt-in Never

Closes #40387
Closes #40420

Change-Id: I2f0810cadcba5f74bdd5a47060ffb14dc8fe7f07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134702
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>

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

@@ -46,6 +46,8 @@ import 'package:kernel/src/bounds_checks.dart'
         findTypeArgumentIssues,
         getGenericTypeName;
 
+import 'package:kernel/src/legacy_erasure.dart';
+
 import 'package:kernel/text/text_serialization_verifier.dart';
 
 import 'package:kernel/type_algebra.dart' show Substitution, substitute;
@@ -1166,6 +1168,11 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
       declaredType = declaredSubstitution.substituteType(declaredType);
     }
 
+    if (!declaredMember.isNonNullableByDefault &&
+        interfaceMember.isNonNullableByDefault) {
+      interfaceType = legacyErasure(types.hierarchy.coreTypes, interfaceType);
+    }
+
     bool inParameter = declaredParameter != null || asIfDeclaredParameter;
     DartType subtype = inParameter ? interfaceType : declaredType;
     DartType supertype = inParameter ? declaredType : interfaceType;

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

@@ -886,7 +886,8 @@ class IsNeverTypeSubtypeOf implements TypeRelation<NeverType> {
 
   IsSubtypeOf isInterfaceRelated(InterfaceType s, NeverType t, Types types) {
     if (s.classNode == types.hierarchy.nullClass) {
-      if (t.nullability == Nullability.nullable) {
+      if (t.nullability == Nullability.nullable ||
+          t.nullability == Nullability.legacy) {
         return const IsSubtypeOf.always();
       }
       if (t.nullability == Nullability.nonNullable) {

pkg/front_end/testcases/nnbd/never_opt_out.dart

@@ -64,6 +64,30 @@ main() {
   throws(() => inferredLocalNever = throwing());
 }
 
+class B extends A {
+  Null neverField;
+  Null neverMethod(Null value) => value;
+  Null get neverProperty => null;
+  void set neverProperty(Null value) {}
+
+  Null nullField;
+  Null nullMethod(Null value) => value;
+  Null get nullProperty => null;
+  void set nullProperty(Null value) {}
+}
+
+class C extends A {
+  Never neverField;
+  Never neverMethod(Never value) => value;
+  Never get neverProperty => null;
+  void set neverProperty(Never value) {}
+
+  Never nullField;
+  Never nullMethod(Never value) => value;
+  Never get nullProperty => null;
+  void set nullProperty(Never value) {}
+}
+
 throws(void Function() f) {
   try {
     f();

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

@@ -1,9 +1,46 @@
 library;
 import self as self;
+import "never_opt_out_lib.dart" as nev;
 import "dart:core" as core;
 
 import "org-dartlang-testcase:///never_opt_out_lib.dart";
 
+class B extends nev::A {
+  field core::Null? neverField;
+  field core::Null? nullField;
+  synthetic constructor •() → self::B*
+    ;
+  method neverMethod(core::Null? value) → core::Null?
+    ;
+  get neverProperty() → core::Null?
+    ;
+  set neverProperty(core::Null? value) → void
+    ;
+  method nullMethod(core::Null? value) → core::Null?
+    ;
+  get nullProperty() → core::Null?
+    ;
+  set nullProperty(core::Null? value) → void
+    ;
+}
+class C extends nev::A {
+  field Never* neverField;
+  field Never* nullField;
+  synthetic constructor •() → self::C*
+    ;
+  method neverMethod(Never* value) → Never*
+    ;
+  get neverProperty() → Never*
+    ;
+  set neverProperty(Never* value) → void
+    ;
+  method nullMethod(Never* value) → Never*
+    ;
+  get nullProperty() → Never*
+    ;
+  set nullProperty(Never* value) → void
+    ;
+}
 static field Never* optOutNever;
 static field core::Null? inferredOptOutNever;
 static method main() → dynamic
@@ -12,10 +49,29 @@ static method throws(() →* void f) → dynamic
   ;
 
 library /*isNonNullableByDefault*/;
-import self as self2;
+import self as nev;
+import "dart:core" as core;
 
 import "org-dartlang-testcase:///never_opt_out.dart";
 
+class A extends core::Object {
+  field Never neverField;
+  field core::Null? nullField;
+  synthetic constructor •() → nev::A
+    ;
+  method neverMethod(Never value) → Never
+    ;
+  get neverProperty() → Never
+    ;
+  set neverProperty(Never value) → void
+    ;
+  method nullMethod(core::Null? value) → core::Null?
+    ;
+  get nullProperty() → core::Null?
+    ;
+  set nullProperty(core::Null? value) → void
+    ;
+}
 static field Never optInNever;
 static method throwing() → Never
   ;

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

@@ -1,10 +1,44 @@
 library;
 import self as self;
-import "dart:core" as core;
 import "never_opt_out_lib.dart" as nev;
+import "dart:core" as core;
 
 import "org-dartlang-testcase:///never_opt_out_lib.dart";
 
+class B extends nev::A {
+  field core::Null? neverField = null;
+  field core::Null? nullField = null;
+  synthetic constructor •() → self::B*
+    : super nev::A::•()
+    ;
+  method neverMethod(core::Null? value) → core::Null?
+    return value;
+  get neverProperty() → core::Null?
+    return null;
+  set neverProperty(core::Null? value) → void {}
+  method nullMethod(core::Null? value) → core::Null?
+    return value;
+  get nullProperty() → core::Null?
+    return null;
+  set nullProperty(core::Null? value) → void {}
+}
+class C extends nev::A {
+  field Never* neverField = null;
+  field Never* nullField = null;
+  synthetic constructor •() → self::C*
+    : super nev::A::•()
+    ;
+  method neverMethod(Never* value) → Never*
+    return value;
+  get neverProperty() → Never*
+    return null;
+  set neverProperty(Never* value) → void {}
+  method nullMethod(Never* value) → Never*
+    return value;
+  get nullProperty() → Never*
+    return null;
+  set nullProperty(Never* value) → void {}
+}
 static field Never* optOutNever;
 static field core::Null? inferredOptOutNever = nev::optInNever;
 static method main() → dynamic {
@@ -65,11 +99,40 @@ static method throws(() →* void f) → dynamic {
 }
 
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_opt_out_lib.dart:19:28: Error: A value of type 'Type' can't be assigned to a variable of type 'Null?'.
+//  - 'Type' is from 'dart:core'.
+//   Null get nullProperty => Null;
+//                            ^
+//
 import self as nev;
+import "dart:core" as core;
 import "never_opt_out.dart" as self;
 
 import "org-dartlang-testcase:///never_opt_out.dart";
 
+class A extends core::Object {
+  field Never neverField = throw "Should not reach here";
+  field core::Null? nullField = null;
+  synthetic constructor •() → nev::A
+    : super core::Object::•()
+    ;
+  method neverMethod(Never value) → Never
+    return value;
+  get neverProperty() → Never
+    return throw "Should not reach here";
+  set neverProperty(Never value) → void {}
+  method nullMethod(core::Null? value) → core::Null?
+    return value;
+  get nullProperty() → core::Null?
+    return let final<BottomType> #t1 = invalid-expression "pkg/front_end/testcases/nnbd/never_opt_out_lib.dart:19:28: Error: A value of type 'Type' can't be assigned to a variable of type 'Null?'.
+ - 'Type' is from 'dart:core'.
+  Null get nullProperty => Null;
+                           ^" in core::Null? as{TypeError,ForNonNullableByDefault} core::Null?;
+  set nullProperty(core::Null? value) → void {}
+}
 static field Never optInNever = self::optOutNever;
 static method throwing() → Never
   return throw "Never!";

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

@@ -1,10 +1,44 @@
 library;
 import self as self;
-import "dart:core" as core;
 import "never_opt_out_lib.dart" as nev;
+import "dart:core" as core;
 
 import "org-dartlang-testcase:///never_opt_out_lib.dart";
 
+class B extends nev::A {
+  field core::Null? neverField = null;
+  field core::Null? nullField = null;
+  synthetic constructor •() → self::B*
+    : super nev::A::•()
+    ;
+  method neverMethod(core::Null? value) → core::Null?
+    return value;
+  get neverProperty() → core::Null?
+    return null;
+  set neverProperty(core::Null? value) → void {}
+  method nullMethod(core::Null? value) → core::Null?
+    return value;
+  get nullProperty() → core::Null?
+    return null;
+  set nullProperty(core::Null? value) → void {}
+}
+class C extends nev::A {
+  field Never* neverField = null;
+  field Never* nullField = null;
+  synthetic constructor •() → self::C*
+    : super nev::A::•()
+    ;
+  method neverMethod(Never* value) → Never*
+    return value;
+  get neverProperty() → Never*
+    return null;
+  set neverProperty(Never* value) → void {}
+  method nullMethod(Never* value) → Never*
+    return value;
+  get nullProperty() → Never*
+    return null;
+  set nullProperty(Never* value) → void {}
+}
 static field Never* optOutNever;
 static field core::Null? inferredOptOutNever = nev::optInNever;
 static method main() → dynamic {
@@ -65,11 +99,37 @@ static method throws(() →* void f) → dynamic {
 }
 
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_opt_out_lib.dart:19:28: Warning: Assigning value of type 'Type' to a variable of type 'Null?'.
+//  - 'Type' is from 'dart:core'.
+//   Null get nullProperty => Null;
+//                            ^
+//
 import self as nev;
+import "dart:core" as core;
 import "never_opt_out.dart" as self;
 
 import "org-dartlang-testcase:///never_opt_out.dart";
 
+class A extends core::Object {
+  field Never neverField = throw "Should not reach here";
+  field core::Null? nullField = null;
+  synthetic constructor •() → nev::A
+    : super core::Object::•()
+    ;
+  method neverMethod(Never value) → Never
+    return value;
+  get neverProperty() → Never
+    return throw "Should not reach here";
+  set neverProperty(Never value) → void {}
+  method nullMethod(core::Null? value) → core::Null?
+    return value;
+  get nullProperty() → core::Null?
+    return core::Null? as{TypeError,ForNonNullableByDefault} core::Null?;
+  set nullProperty(core::Null? value) → void {}
+}
 static field Never optInNever = self::optOutNever;
 static method throwing() → Never
   return throw "Never!";

pkg/front_end/testcases/nnbd/never_opt_out_lib.dart

@@ -7,3 +7,15 @@ import 'never_opt_out.dart';
 Never throwing() => throw 'Never!';
 
 Never optInNever = optOutNever;
+
+class A {
+  Never neverField = throw "Should not reach here";
+  Never neverMethod(Never value) => value;
+  Never get neverProperty => throw "Should not reach here";
+  void set neverProperty(Never value) {}
+
+  Null nullField = null;
+  Null nullMethod(Null value) => value;
+  Null get nullProperty => Null;
+  void set nullProperty(Null value) {}
+}

pkg/front_end/testcases/outline.status

@@ -55,6 +55,7 @@ inference_new/infer_field_override_getter_overrides_setter: TypeCheckError
 nnbd/inheritance_from_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_out: TypeCheckError
+nnbd/never_opt_out: TypeCheckError
 
 rasta/native_is_illegal: Pass # Issue 29763
 

pkg/front_end/testcases/strong.status

@@ -171,6 +171,7 @@ instantiate_to_bound/non_simple_generic_function_in_bound_regress: RuntimeError
 nnbd/inheritance_from_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_out: TypeCheckError
+nnbd/never_opt_out: TypeCheckError
 rasta/abstract_constructor: RuntimeError
 rasta/bad_constructor_redirection: RuntimeError
 rasta/bad_continue: RuntimeError

pkg/front_end/testcases/text_serialization.status

@@ -1246,7 +1246,7 @@ nnbd/messages_with_types_opt_out: TypeCheckError
 nnbd/missing_required_named_parameter: TextSerializationFailure
 nnbd/mixin_from_opt_in: TextSerializationFailure
 nnbd/mixin_from_opt_out: TextSerializationFailure
-nnbd/never_opt_out: TextSerializationFailure
+nnbd/never_opt_out: TypeCheckError
 nnbd/never_receiver: TextSerializationFailure
 nnbd/nnbd_opt_out_language_version: TextSerializationFailure
 nnbd/nnbd_opt_out_language_version_try_to_trick: TextSerializationFailure

pkg/front_end/testcases/weak.status

@@ -65,4 +65,5 @@ general_nnbd_opt_out/void_methods: RuntimeError
 general_nnbd_opt_out/warn_unresolved_sends: InstrumentationMismatch # Test assumes Dart 1.0 semantics
 nnbd/inheritance_from_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_in: TypeCheckError
-nnbd/messages_with_types_opt_out: TypeCheckError
+nnbd/messages_with_types_opt_out: TypeCheckError
+nnbd/never_opt_out: TypeCheckError