Extension types. It is not an error to invoke members of nullable extension type.

1. It is not allowed to write a nullable type in `implements`.
2. Any methods of the extension type itself are OK to invoke.

I start to doubt what is the meaning of isNonNullable() to extension types.
For now I will add a workaround, to be reconsidered if isNonNullable() is
clarified.

Change-Id: I8a9543a6b0e942ae4de3ce673eeee1aa9b0a5230
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/319862
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/element/extensions.dart

@@ -11,6 +11,10 @@ import 'package:collection/collection.dart';
 import 'package:meta/meta_meta.dart';
 
 extension DartTypeExtension on DartType {
+  bool get isExtensionType {
+    return element is ExtensionTypeElement;
+  }
+
   /// If `this` is an [InterfaceType] that is an instantiation of an extension
   /// type, returns its representation type erasure. Otherwise, returns self.
   DartType get representationTypeErasureOrSelf {

pkg/analyzer/lib/src/dart/resolver/type_property_resolver.dart

@@ -92,7 +92,8 @@ class TypePropertyResolver {
     }
 
     if (_isNonNullableByDefault &&
-        _typeSystem.isPotentiallyNullable(receiverType)) {
+        _typeSystem.isPotentiallyNullable(receiverType) &&
+        !receiverType.isExtensionType) {
       _lookupInterfaceType(_typeProvider.objectType);
       if (_hasGetterOrSetter) {
         return _toResult();

pkg/analyzer/test/src/dart/resolution/method_invocation_test.dart

@@ -817,6 +817,37 @@ extension type A(int it) {
   void foo() {}
 }
 
+void f(A a) {
+  a.foo();
+}
+''');
+
+    final node = findNode.singleMethodInvocation;
+    assertResolvedNodeText(node, r'''
+MethodInvocation
+  target: SimpleIdentifier
+    token: a
+    staticElement: self::@function::f::@parameter::a
+    staticType: A
+  operator: .
+  methodName: SimpleIdentifier
+    token: foo
+    staticElement: self::@extensionType::A::@method::foo
+    staticType: void Function()
+  argumentList: ArgumentList
+    leftParenthesis: (
+    rightParenthesis: )
+  staticInvokeType: void Function()
+  staticType: void
+''');
+  }
+
+  test_hasReceiver_interfaceType_extensionType_declared_nullableRepresentation() async {
+    await assertNoErrorsInCode(r'''
+extension type A(int? it) {
+  void foo() {}
+}
+
 void f(A a) {
   a.foo();
 }

pkg/analyzer/test/src/dart/resolution/prefixed_identifier_test.dart

@@ -549,6 +549,34 @@ extension type A(int it) {
   int get foo => 0;
 }
 
+void f(A a) {
+  a.foo;
+}
+''');
+
+    final node = findNode.singlePrefixedIdentifier;
+    assertResolvedNodeText(node, r'''
+PrefixedIdentifier
+  prefix: SimpleIdentifier
+    token: a
+    staticElement: self::@function::f::@parameter::a
+    staticType: A
+  period: .
+  identifier: SimpleIdentifier
+    token: foo
+    staticElement: self::@extensionType::A::@getter::foo
+    staticType: int
+  staticElement: self::@extensionType::A::@getter::foo
+  staticType: int
+''');
+  }
+
+  test_ofExtensionType_read_nullableRepresentation() async {
+    await assertNoErrorsInCode(r'''
+extension type A(int? it) {
+  int get foo => 0;
+}
+
 void f(A a) {
   a.foo;
 }