Issue 40931. Update inference constraint for T? to be non-nullable.

Bug: #40931
Change-Id: I483c3febad236b366f911f38339c712dd928cfcf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138790
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/generated/type_system.dart

@@ -649,6 +649,9 @@ class GenericInferrer {
       var constraints = this.constraints[t2.element];
       if (constraints != null) {
         if (!identical(t1, UnknownInferredType.instance)) {
+          if (t2.nullabilitySuffix == NullabilitySuffix.question) {
+            t1 = _typeSystem.promoteToNonNull(t1);
+          }
           var constraint = _TypeConstraint(origin, t2.element, lower: t1);
           constraints.add(constraint);
           _undoBuffer.add(constraint);

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

@@ -1702,6 +1702,9 @@ class MethodInvocationResolutionWithNnbdTest extends DriverResolutionTest {
       [EnableString.non_nullable],
     );
 
+  @override
+  bool get typeToStringWithNullability => true;
+
   test_hasReceiver_deferredImportPrefix_loadLibrary_optIn_fromOptOut() async {
     newFile('/test/lib/a.dart', content: r'''
 class A {}
@@ -1724,7 +1727,7 @@ main() {
     assertMethodInvocation(
       invocation,
       import.importedLibrary.loadLibraryFunction,
-      'Future<dynamic> Function()',
+      'Future<dynamic>* Function()*',
     );
   }
 
@@ -1834,6 +1837,29 @@ main(A? a) {
     );
   }
 
+  test_hasReceiver_interfaceTypeQ_defined_extensionQ2() async {
+    await assertNoErrorsInCode(r'''
+extension E<T> on T? {
+  T foo() => throw 0;
+}
+
+main(int? a) {
+  a.foo();
+}
+''');
+
+    assertMethodInvocation2(
+      findNode.methodInvocation('a.foo()'),
+      element: elementMatcher(
+        findElement.method('foo', of: 'E'),
+        substitution: {'T': 'int'},
+      ),
+      typeArgumentTypes: [],
+      invokeType: 'int Function()',
+      type: 'int',
+    );
+  }
+
   test_hasReceiver_interfaceTypeQ_notDefined() async {
     await assertErrorsInCode(r'''
 class A {}

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

@@ -454,7 +454,7 @@ mixin ResolutionTest implements ResourceProviderMixin {
 
   void assertMethodInvocation2(
     MethodInvocation node, {
-    @required ExecutableElement element,
+    @required Object element,
     @required List<String> typeArgumentTypes,
     @required String invokeType,
     @required String type,

tests/language/nnbd/resolution/issue40931_test.dart

@@ -0,0 +1,15 @@
+// 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.
+
+// SharedOptions=--enable-experiment=non-nullable
+
+main() {}
+
+extension E<T> on T? {
+  T foo() => throw 0;
+}
+
+void f(int? a) {
+  a.foo();
+}