Add "why not promoted" support for explicit extension overrides.

Bug: #44898
Change-Id: Iea9300cf2cae52355bfa5a71f773c8136da2c644
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/195306
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/why_not_promoted/data/argument_type_not_assignable_nullability_error.dart

@@ -473,3 +473,19 @@ indexSetPostIncDec(C32 c, List<int> values) {
       /*analyzer.notPromoted(propertyNotPromoted(target: member:C32.bad, type: int?))*/ c
           . /*cfe.notPromoted(propertyNotPromoted(target: member:C32.bad, type: int?))*/ bad]--;
 }
+
+extension E33 on int {
+  void f() {}
+}
+
+class C33 {
+  int? bad;
+}
+
+test(C33 c) {
+  if (c.bad == null) return;
+  E33(
+          /*analyzer.notPromoted(propertyNotPromoted(target: member:C33.bad, type: int?))*/ c
+              . /*cfe.notPromoted(propertyNotPromoted(target: member:C33.bad, type: int?))*/ bad)
+      .f();
+}

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

@@ -2,6 +2,7 @@
 // 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 'package:_fe_analyzer_shared/src/flow_analysis/flow_analysis.dart';
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/syntactic_entity.dart';
@@ -119,7 +120,8 @@ class ExtensionMemberResolver {
   }
 
   /// Perform upward inference for the override.
-  void resolveOverride(ExtensionOverride node) {
+  void resolveOverride(ExtensionOverride node,
+      List<Map<DartType, NonPromotionReason> Function()> whyNotPromotedList) {
     var nodeImpl = node as ExtensionOverrideImpl;
     var element = node.staticElement!;
     var typeParameters = element.typeParameters;
@@ -173,10 +175,14 @@ class ExtensionMemberResolver {
       _errorReporter.reportErrorForNode(
           CompileTimeErrorCode.USE_OF_VOID_RESULT, receiverExpression);
     } else if (!_typeSystem.isAssignableTo(receiverType, node.extendedType!)) {
+      var whyNotPromoted =
+          whyNotPromotedList.isEmpty ? null : whyNotPromotedList[0];
       _errorReporter.reportErrorForNode(
         CompileTimeErrorCode.EXTENSION_OVERRIDE_ARGUMENT_NOT_ASSIGNABLE,
         receiverExpression,
         [receiverType, node.extendedType],
+        _resolver.computeWhyNotPromotedMessages(
+            receiverExpression, whyNotPromoted?.call()),
       );
     }
   }

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

@@ -1456,7 +1456,7 @@ class ResolverVisitor extends ScopedVisitor with ErrorDetectionHelpers {
         whyNotPromotedList: whyNotPromotedList);
 
     node.accept(elementResolver);
-    node.accept(typeAnalyzer);
+    extensionResolver.resolveOverride(node, whyNotPromotedList);
   }
 
   @override

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

@@ -128,7 +128,8 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<void> {
 
   @override
   void visitExtensionOverride(ExtensionOverride node) {
-    _resolver.extensionResolver.resolveOverride(node);
+    assert(false,
+        'Resolver should call extensionResolver.resolveOverride directly');
   }
 
   /// The Dart Language Specification, 12.9: <blockquote>The static type of a function literal of the