[cfe] Allow member access on Never

Closes #40125.

Bug: http://dartbug.com/40125
Change-Id: I2dcb1bdc022990ba8c76c7798c4e6793c5c89108
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134326
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/fasta/fasta_codes_cfe_generated.dart

@@ -1520,6 +1520,42 @@ Message _withArgumentsInitializingFormalTypeMismatch(
       arguments: {'name': name, 'type': _type, 'type2': _type2});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<
+        Message Function(
+            String string, DartType _type, bool isNonNullableByDefault)>
+    templateInternalProblemUnsupportedNullability = const Template<
+            Message Function(
+                String string, DartType _type, bool isNonNullableByDefault)>(
+        messageTemplate:
+            r"""Unsupported nullability value '#string' on type '#type'.""",
+        withArguments: _withArgumentsInternalProblemUnsupportedNullability);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<
+        Message Function(
+            String string, DartType _type, bool isNonNullableByDefault)>
+    codeInternalProblemUnsupportedNullability = const Code<
+            Message Function(
+                String string, DartType _type, bool isNonNullableByDefault)>(
+        "InternalProblemUnsupportedNullability",
+        templateInternalProblemUnsupportedNullability,
+        severity: Severity.internalProblem);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsInternalProblemUnsupportedNullability(
+    String string, DartType _type, bool isNonNullableByDefault) {
+  if (string.isEmpty) throw 'No string provided';
+  TypeLabeler labeler = new TypeLabeler(isNonNullableByDefault);
+  List<Object> typeParts = labeler.labelType(_type);
+  String type = typeParts.join();
+  return new Message(codeInternalProblemUnsupportedNullability,
+      message:
+          """Unsupported nullability value '${string}' on type '${type}'.""" +
+              labeler.originMessages,
+      arguments: {'string': string, 'type': _type});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
         Message Function(

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

@@ -48,7 +48,7 @@ import '../kernel/type_algorithms.dart' show hasAnyTypeVariables;
 
 import '../names.dart';
 
-import '../problems.dart' show unexpected, unhandled;
+import '../problems.dart' show internalProblem, unexpected, unhandled;
 
 import '../source/source_library_builder.dart' show SourceLibraryBuilder;
 
@@ -1051,6 +1051,24 @@ class TypeInferrerImpl implements TypeInferrer {
       target = new ObjectAccessTarget.interfaceMember(interfaceMember);
     } else if (receiverType is DynamicType) {
       target = const ObjectAccessTarget.dynamic();
+    } else if (receiverType is NeverType) {
+      switch (receiverType.nullability) {
+        case Nullability.nonNullable:
+          target = const ObjectAccessTarget.never();
+          break;
+        case Nullability.nullable:
+        case Nullability.legacy:
+          // Never? and Never* are equivalent to Null.
+          return findInterfaceMember(coreTypes.nullType, name, fileOffset);
+        case Nullability.undetermined:
+          return internalProblem(
+              templateInternalProblemUnsupportedNullability.withArguments(
+                  "${receiverType.nullability}",
+                  receiverType,
+                  isNonNullableByDefault),
+              fileOffset,
+              library.fileUri);
+      }
     } else if (receiverType is InvalidType) {
       target = const ObjectAccessTarget.invalid();
     } else if (receiverType is InterfaceType &&
@@ -1356,6 +1374,8 @@ class TypeInferrerImpl implements TypeInferrer {
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         return const DynamicType();
+      case ObjectAccessTargetKind.never:
+        return const NeverType(Nullability.nonNullable);
       case ObjectAccessTargetKind.instanceMember:
         return getGetterTypeForMemberTarget(target.member, receiverType);
       case ObjectAccessTargetKind.extensionMember:
@@ -1456,6 +1476,7 @@ class TypeInferrerImpl implements TypeInferrer {
         return _getFunctionType(receiverType);
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
+      case ObjectAccessTargetKind.never:
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         return unknownFunction;
@@ -1519,6 +1540,8 @@ class TypeInferrerImpl implements TypeInferrer {
             throw unhandled('$target', 'getFunctionType', null, null);
         }
         break;
+      case ObjectAccessTargetKind.never:
+        return const NeverType(Nullability.nonNullable);
       case ObjectAccessTargetKind.callFunction:
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
@@ -1556,6 +1579,7 @@ class TypeInferrerImpl implements TypeInferrer {
       case ObjectAccessTargetKind.callFunction:
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
+      case ObjectAccessTargetKind.never:
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         break;
@@ -1614,6 +1638,7 @@ class TypeInferrerImpl implements TypeInferrer {
       case ObjectAccessTargetKind.callFunction:
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
+      case ObjectAccessTargetKind.never:
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         break;
@@ -1669,6 +1694,7 @@ class TypeInferrerImpl implements TypeInferrer {
       case ObjectAccessTargetKind.callFunction:
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
+      case ObjectAccessTargetKind.never:
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         break;
@@ -1723,6 +1749,7 @@ class TypeInferrerImpl implements TypeInferrer {
     switch (target.kind) {
       case ObjectAccessTargetKind.unresolved:
       case ObjectAccessTargetKind.dynamic:
+      case ObjectAccessTargetKind.never:
       case ObjectAccessTargetKind.invalid:
       case ObjectAccessTargetKind.missing:
         return const DynamicType();
@@ -2651,6 +2678,25 @@ class TypeInferrerImpl implements TypeInferrer {
         nullAwareGuards);
   }
 
+  ExpressionInferenceResult _inferNeverInvocation(
+      int fileOffset,
+      Link<NullAwareGuard> nullAwareGuards,
+      Expression receiver,
+      NeverType receiverType,
+      Name name,
+      Arguments arguments,
+      DartType typeContext) {
+    InvocationInferenceResult result = inferInvocation(
+        typeContext, fileOffset, unknownFunction, arguments, name,
+        receiverType: receiverType);
+    assert(name != equalsName);
+    return createNullAwareExpressionInferenceResult(
+        result.inferredType,
+        result.applyResult(new MethodInvocation(receiver, name, arguments)
+          ..fileOffset = fileOffset),
+        nullAwareGuards);
+  }
+
   ExpressionInferenceResult _inferMissingInvocation(
       int fileOffset,
       Link<NullAwareGuard> nullAwareGuards,
@@ -3046,6 +3092,9 @@ class TypeInferrerImpl implements TypeInferrer {
       case ObjectAccessTargetKind.unresolved:
         return _inferDynamicInvocation(fileOffset, nullAwareGuards, receiver,
             name, arguments, typeContext);
+      case ObjectAccessTargetKind.never:
+        return _inferNeverInvocation(fileOffset, nullAwareGuards, receiver,
+            receiverType, name, arguments, typeContext);
     }
     return unhandled(
         '$target', 'inferMethodInvocation', fileOffset, uriForInstrumentation);
@@ -3931,6 +3980,7 @@ enum ObjectAccessTargetKind {
   callFunction,
   extensionMember,
   dynamic,
+  never,
   invalid,
   missing,
   // TODO(johnniwinther): Remove this.
@@ -3972,6 +4022,10 @@ class ObjectAccessTarget {
   const ObjectAccessTarget.dynamic()
       : this.internal(ObjectAccessTargetKind.dynamic, null);
 
+  /// Creates an access on a receiver of type Never with no known target.
+  const ObjectAccessTarget.never()
+      : this.internal(ObjectAccessTargetKind.never, null);
+
   /// Creates an access with no target due to an invalid receiver type.
   ///
   /// This is not in itself an error but a consequence of another error.

pkg/front_end/messages.yaml

@@ -1704,6 +1704,10 @@ InternalProblemUnfinishedTypeVariable:
   template: "Unfinished type variable '#name' found in non-source library '#uri'."
   severity: INTERNAL_PROBLEM
 
+InternalProblemUnsupportedNullability:
+  template: "Unsupported nullability value '#string' on type '#type'."
+  severity: INTERNAL_PROBLEM
+
 LocalDefinitionHidesExport:
   template: "Local definition of '#name' hides export from '#uri'."
   severity: IGNORED

pkg/front_end/test/spell_checking_list_common.txt

@@ -1912,6 +1912,7 @@ noticed
 notifies
 now
 null
+nullability
 nullable
 num
 number

pkg/front_end/testcases/nnbd/never_receiver.dart

@@ -0,0 +1,34 @@
+// 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.
+
+foo(Never x, Never? y) {
+  var local0 = y.toString(); // Not an error.
+  var local1 = y.hashCode; // Not an error.
+
+  x.foo(); // Not an error.
+  x.bar; // Not an error.
+  x.baz = 42; // Not an error.
+  x(); // Not an error.
+  x[42]; // Not an error.
+  x[42] = 42; // Not an error.
+  x++; // Not an error.
+  x += 1; // Not an error.
+  y?.foo(); // Not an error.
+  y?.bar; // Not an error.
+  y?.baz = 42; // Not an error.
+  y?.call(); // Not an error.
+  y?.[42]; // Not an error.
+  y?.[42] = 42; // Not an error.
+
+  y.foo(); // Error.
+  y.bar; // Error.
+  y.baz = 42; // Error.
+  y(); // Error.
+  y++; // Error.
+  y += 1; // Error.
+  y[42]; // Error.
+  y[42] = 42; // Error.
+}
+
+main() {}

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

@@ -0,0 +1,7 @@
+library;
+import self as self;
+
+static method foo(Neverx, Never? y) → dynamic
+  ;
+static method main() → dynamic
+  ;

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

@@ -0,0 +1,146 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: Property 'bar' cannot be accessed on 'Never' because it is potentially null.
+// Try accessing using ?. instead.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: Property 'baz' cannot be accessed on 'Never' because it is potentially null.
+// Try accessing using ?. instead.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'call'.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: Can't use an expression of type 'Never' as a function because it's potentially null.
+// Try calling using ?.call instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]'.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: Operator '[]' cannot be called on 'Never' because it is potentially null.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]='.
+//   y[42] = 42; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: Operator '[]=' cannot be called on 'Never' because it is potentially null.
+//   y[42] = 42; // Error.
+//    ^
+//
+import self as self;
+import "dart:core" as core;
+
+static method foo(Neverx, Never? y) → dynamic {
+  core::String local0 = y.{core::Object::toString}();
+  core::int local1 = y.{core::Object::hashCode};
+  x.foo();
+  x.bar;
+  x.baz = 42;
+  x.call();
+  x.[](42);
+  x.[]=(42, 42);
+  x = x.+(1);
+  x = x.+(1);
+  let finalNever? #t1 = y in #t1.{core::Object::==}(null) ?{dynamic} null : #t1{Never}.foo();
+  let finalNever? #t2 = y in #t2.{core::Object::==}(null) ?{core::Null?} null : #t2{Never}.bar;
+  let finalNever? #t3 = y in #t3.{core::Object::==}(null) ?{core::int?} null : #t3{Never}.baz = 42;
+  let finalNever? #t4 = y in #t4.{core::Object::==}(null) ?{dynamic} null : #t4{Never}.call();
+  let finalNever? #t5 = y in #t5.{core::Object::==}(null) ?{core::Null?} null : #t5{Never}.[](42);
+  let finalNever? #t6 = y in #t6.{core::Object::==}(null) ?{core::int?} null : #t6{Never}.[]=(42, 42);
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+  y.foo(); // Error.
+    ^^^";
+  let final<BottomType> #t7 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: Property 'bar' cannot be accessed on 'Never' because it is potentially null.
+Try accessing using ?. instead.
+  y.bar; // Error.
+    ^^^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+  y.bar; // Error.
+    ^^^";
+  let final<BottomType> #t8 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: Property 'baz' cannot be accessed on 'Never' because it is potentially null.
+Try accessing using ?. instead.
+  y.baz = 42; // Error.
+    ^^^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+  y.baz = 42; // Error.
+    ^^^";
+  let final<BottomType> #t9 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: Can't use an expression of type 'Never' as a function because it's potentially null.
+Try calling using ?.call instead.
+  y(); // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'call'.
+  y(); // Error.
+   ^";
+  y = (let final<BottomType> #t10 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+  y++; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y++; // Error.
+   ^") as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  y = (let final<BottomType> #t11 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+  y += 1; // Error.
+    ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y += 1; // Error.
+    ^") as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  let final<BottomType> #t12 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: Operator '[]' cannot be called on 'Never' because it is potentially null.
+  y[42]; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]'.
+  y[42]; // Error.
+   ^";
+  let final<BottomType> #t13 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: Operator '[]=' cannot be called on 'Never' because it is potentially null.
+  y[42] = 42; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]='.
+  y[42] = 42; // Error.
+   ^";
+}
+static method main() → dynamic {}

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

@@ -0,0 +1,146 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: Property 'bar' cannot be accessed on 'Never' because it is potentially null.
+// Try accessing using ?. instead.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: Property 'baz' cannot be accessed on 'Never' because it is potentially null.
+// Try accessing using ?. instead.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'call'.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: Can't use an expression of type 'Never' as a function because it's potentially null.
+// Try calling using ?.call instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]'.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: Operator '[]' cannot be called on 'Never' because it is potentially null.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]='.
+//   y[42] = 42; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: Operator '[]=' cannot be called on 'Never' because it is potentially null.
+//   y[42] = 42; // Error.
+//    ^
+//
+import self as self;
+import "dart:core" as core;
+
+static method foo(Neverx, Never? y) → dynamic {
+  core::String local0 = y.{core::Object::toString}();
+  core::int local1 = y.{core::Object::hashCode};
+  x.foo();
+  x.bar;
+  x.baz = 42;
+  x.call();
+  x.[](42);
+  x.[]=(42, 42);
+  x = x.+(1);
+  x = x.+(1);
+  let finalNever? #t1 = y in #t1.{core::Object::==}(null) ?{dynamic} null : #t1{Never}.foo();
+  let finalNever? #t2 = y in #t2.{core::Object::==}(null) ?{core::Null?} null : #t2{Never}.bar;
+  let finalNever? #t3 = y in #t3.{core::Object::==}(null) ?{core::int?} null : #t3{Never}.baz = 42;
+  let finalNever? #t4 = y in #t4.{core::Object::==}(null) ?{dynamic} null : #t4{Never}.call();
+  let finalNever? #t5 = y in #t5.{core::Object::==}(null) ?{core::Null?} null : #t5{Never}.[](42);
+  let finalNever? #t6 = y in #t6.{core::Object::==}(null) ?{core::int?} null : #t6{Never}.[]=(42, 42);
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+  y.foo(); // Error.
+    ^^^";
+  let final<BottomType> #t7 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: Property 'bar' cannot be accessed on 'Never' because it is potentially null.
+Try accessing using ?. instead.
+  y.bar; // Error.
+    ^^^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+  y.bar; // Error.
+    ^^^";
+  let final<BottomType> #t8 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: Property 'baz' cannot be accessed on 'Never' because it is potentially null.
+Try accessing using ?. instead.
+  y.baz = 42; // Error.
+    ^^^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+  y.baz = 42; // Error.
+    ^^^";
+  let final<BottomType> #t9 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: Can't use an expression of type 'Never' as a function because it's potentially null.
+Try calling using ?.call instead.
+  y(); // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'call'.
+  y(); // Error.
+   ^";
+  y = (let final<BottomType> #t10 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+  y++; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y++; // Error.
+   ^") as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  y = (let final<BottomType> #t11 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: Operator '+' cannot be called on 'Never' because it is potentially null.
+  y += 1; // Error.
+    ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y += 1; // Error.
+    ^") as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  let final<BottomType> #t12 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: Operator '[]' cannot be called on 'Never' because it is potentially null.
+  y[42]; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]'.
+  y[42]; // Error.
+   ^";
+  let final<BottomType> #t13 = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: Operator '[]=' cannot be called on 'Never' because it is potentially null.
+  y[42] = 42; // Error.
+   ^" in invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]='.
+  y[42] = 42; // Error.
+   ^";
+}
+static method main() → dynamic {}

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

@@ -0,0 +1,139 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Warning: Method 'foo' is called on 'Never' which is potentially null.
+// Try calling using ?. instead.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Warning: Property 'bar' is accessed on 'Never' which is potentially null.
+// Try accessing using ?. instead.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Warning: Property 'baz' is accessed on 'Never' which is potentially null.
+// Try accessing using ?. instead.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'call'.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Warning: Method 'call' is called on 'Never' which is potentially null.
+// Try calling using ?. instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Warning: Expression of type 'Never' is used as a function, but it's potentially null.
+// Try calling using ?.call instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Warning: Operator '+' is called on 'Never' which is potentially null.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Warning: Operator '+' is called on 'Never' which is potentially null.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]'.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Warning: Operator '[]' is called on 'Never' which is potentially null.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]='.
+//   y[42] = 42; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Warning: Operator '[]=' is called on 'Never' which is potentially null.
+//   y[42] = 42; // Error.
+//    ^
+//
+import self as self;
+import "dart:core" as core;
+
+static method foo(Neverx, Never? y) → dynamic {
+  core::String local0 = y.{core::Object::toString}();
+  core::int local1 = y.{core::Object::hashCode};
+  x.foo();
+  x.bar;
+  x.baz = 42;
+  x.call();
+  x.[](42);
+  x.[]=(42, 42);
+  x = x.+(1);
+  x = x.+(1);
+  let finalNever? #t1 = y in #t1.{core::Object::==}(null) ?{dynamic} null : #t1{Never}.foo();
+  let finalNever? #t2 = y in #t2.{core::Object::==}(null) ?{core::Null?} null : #t2{Never}.bar;
+  let finalNever? #t3 = y in #t3.{core::Object::==}(null) ?{core::int?} null : #t3{Never}.baz = 42;
+  let finalNever? #t4 = y in #t4.{core::Object::==}(null) ?{dynamic} null : #t4{Never}.call();
+  let finalNever? #t5 = y in #t5.{core::Object::==}(null) ?{core::Null?} null : #t5{Never}.[](42);
+  let finalNever? #t6 = y in #t6.{core::Object::==}(null) ?{core::int?} null : #t6{Never}.[]=(42, 42);
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+  y.foo(); // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+  y.bar; // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+  y.baz = 42; // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'call'.
+  y(); // Error.
+   ^";
+  y = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y++; // Error.
+   ^" as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  y = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y += 1; // Error.
+    ^" as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]'.
+  y[42]; // Error.
+   ^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]='.
+  y[42] = 42; // Error.
+   ^";
+}
+static method main() → dynamic {}

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

@@ -0,0 +1,139 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Warning: Method 'foo' is called on 'Never' which is potentially null.
+// Try calling using ?. instead.
+//   y.foo(); // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Warning: Property 'bar' is accessed on 'Never' which is potentially null.
+// Try accessing using ?. instead.
+//   y.bar; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Warning: Property 'baz' is accessed on 'Never' which is potentially null.
+// Try accessing using ?. instead.
+//   y.baz = 42; // Error.
+//     ^^^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named 'call'.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Warning: Method 'call' is called on 'Never' which is potentially null.
+// Try calling using ?. instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Warning: Expression of type 'Never' is used as a function, but it's potentially null.
+// Try calling using ?.call instead.
+//   y(); // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Warning: Operator '+' is called on 'Never' which is potentially null.
+//   y++; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '+'.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Warning: Operator '+' is called on 'Never' which is potentially null.
+//   y += 1; // Error.
+//     ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]'.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Warning: Operator '[]' is called on 'Never' which is potentially null.
+//   y[42]; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+// Try correcting the name to the name of an existing method, or defining a method named '[]='.
+//   y[42] = 42; // Error.
+//    ^
+//
+// pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Warning: Operator '[]=' is called on 'Never' which is potentially null.
+//   y[42] = 42; // Error.
+//    ^
+//
+import self as self;
+import "dart:core" as core;
+
+static method foo(Neverx, Never? y) → dynamic {
+  core::String local0 = y.{core::Object::toString}();
+  core::int local1 = y.{core::Object::hashCode};
+  x.foo();
+  x.bar;
+  x.baz = 42;
+  x.call();
+  x.[](42);
+  x.[]=(42, 42);
+  x = x.+(1);
+  x = x.+(1);
+  let finalNever? #t1 = y in #t1.{core::Object::==}(null) ?{dynamic} null : #t1{Never}.foo();
+  let finalNever? #t2 = y in #t2.{core::Object::==}(null) ?{core::Null?} null : #t2{Never}.bar;
+  let finalNever? #t3 = y in #t3.{core::Object::==}(null) ?{core::int?} null : #t3{Never}.baz = 42;
+  let finalNever? #t4 = y in #t4.{core::Object::==}(null) ?{dynamic} null : #t4{Never}.call();
+  let finalNever? #t5 = y in #t5.{core::Object::==}(null) ?{core::Null?} null : #t5{Never}.[](42);
+  let finalNever? #t6 = y in #t6.{core::Object::==}(null) ?{core::int?} null : #t6{Never}.[]=(42, 42);
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:24:5: Error: The method 'foo' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'foo'.
+  y.foo(); // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:25:5: Error: The getter 'bar' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'bar'.
+  y.bar; // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:26:5: Error: The setter 'baz' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing setter, or defining a setter or field named 'baz'.
+  y.baz = 42; // Error.
+    ^^^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:27:4: Error: The method 'call' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named 'call'.
+  y(); // Error.
+   ^";
+  y = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:28:4: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y++; // Error.
+   ^" as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  y = invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:29:5: Error: The method '+' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '+'.
+  y += 1; // Error.
+    ^" as{TypeError,ForDynamic,ForNonNullableByDefault} Never?;
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:30:4: Error: The method '[]' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]'.
+  y[42]; // Error.
+   ^";
+  invalid-expression "pkg/front_end/testcases/nnbd/never_receiver.dart:31:4: Error: The method '[]=' isn't defined for the class 'Never'.
+Try correcting the name to the name of an existing method, or defining a method named '[]='.
+  y[42] = 42; // Error.
+   ^";
+}
+static method main() → dynamic {}

pkg/front_end/testcases/text_serialization.status

@@ -1241,6 +1241,7 @@ nnbd/member_inheritance_from_opt_out: TextSerializationFailure
 nnbd/messages_with_types_opt_in: TypeCheckError
 nnbd/messages_with_types_opt_out: TypeCheckError
 nnbd/missing_required_named_parameter: TextSerializationFailure
+nnbd/never_receiver: TextSerializationFailure
 nnbd/nnbd_opt_out_language_version: TextSerializationFailure
 nnbd/nnbd_opt_out_language_version_try_to_trick: TextSerializationFailure
 nnbd/no_null_shorting: TextSerializationFailure

pkg/kernel/lib/naive_type_checker.dart

@@ -249,6 +249,10 @@ super method declares ${superParameter.type}
     if (receiver is DynamicType) {
       return;
     }
+    if (receiver is NeverType &&
+        receiver.nullability == Nullability.nonNullable) {
+      return;
+    }
 
     // Permit any invocation on Function type.
     if (receiver == environment.coreTypes.functionLegacyRawType &&