Analyzer: tidy function reference parsing.

Check for odd case of attempting to type instantiate an extension (it's not supported).

Re-write some TODOs as comments.

Change-Id: I14fd2a4e21139939ca60e415be9ac31b93ea50ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213240
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

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

@@ -387,7 +387,8 @@ class FunctionReferenceResolver {
         );
 
         if (methodElement == null) {
-          // TODO(srawlins): Can we get here?
+          // The target is known, but the method is not; [UNDEFINED_GETTER] is
+          // reported elsewhere.
           node.staticType = DynamicTypeImpl.instance;
           return;
         } else {
@@ -395,7 +396,7 @@ class FunctionReferenceResolver {
           return;
         }
       } else {
-        // TODO(srawlins): Can we get here?
+        // The prefix is unknown; [UNDEFINED_IDENTIFIER] is reported elsewhere.
         node.staticType = DynamicTypeImpl.instance;
         return;
       }
@@ -440,7 +441,7 @@ class FunctionReferenceResolver {
   void _resolveReceiverPrefix(
     FunctionReferenceImpl node,
     PrefixElement prefixElement,
-    PrefixedIdentifier prefix,
+    PrefixedIdentifierImpl prefix,
     Element element,
   ) {
     if (element is MultiplyDefinedElement) {
@@ -477,10 +478,19 @@ class FunctionReferenceResolver {
         name: element.name,
       );
       return;
+    } else if (element is ExtensionElement) {
+      prefix.identifier.staticElement = element;
+      prefix.identifier.staticType = DynamicTypeImpl.instance;
+      prefix.staticType = DynamicTypeImpl.instance;
+      _resolveDisallowedExpression(node, DynamicTypeImpl.instance);
+      return;
     }
 
-    // TODO(srawlins): Report undefined prefixed identifier.
-
+    assert(
+      false,
+      'Member of prefixed element, $prefixElement, is not a class, mixin, '
+      'type alias, or executable element: $element (${element.runtimeType})',
+    );
     node.staticType = DynamicTypeImpl.instance;
   }
 
@@ -590,6 +600,11 @@ class FunctionReferenceResolver {
       function.staticType = element.type;
       _resolve(node: node, rawType: element.type);
       return;
+    } else if (element is ExtensionElement) {
+      function.staticElement = element;
+      function.staticType = DynamicTypeImpl.instance;
+      _resolveDisallowedExpression(node, DynamicTypeImpl.instance);
+      return;
     } else {
       _resolveDisallowedExpression(node, DynamicTypeImpl.instance);
       return;

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

@@ -109,6 +109,50 @@ bar() {
         findNode.functionReference('foo<int>;'), null, 'dynamic');
   }
 
+  test_extension() async {
+    await assertErrorsInCode('''
+extension E<T> on String {}
+
+void foo() {
+  E<int>;
+}
+''', [
+      error(
+          CompileTimeErrorCode.DISALLOWED_TYPE_INSTANTIATION_EXPRESSION, 44, 1),
+    ]);
+
+    var reference = findNode.functionReference('E<int>;');
+    assertFunctionReference(
+      reference,
+      findElement.extension_('E'),
+      'dynamic',
+    );
+  }
+
+  test_extension_prefixed() async {
+    newFile('$testPackageLibPath/a.dart', content: '''
+extension E<T> on String {}
+''');
+    await assertErrorsInCode('''
+import 'a.dart' as a;
+
+void foo() {
+  a.E<int>;
+}
+''', [
+      error(
+          CompileTimeErrorCode.DISALLOWED_TYPE_INSTANTIATION_EXPRESSION, 38, 3),
+    ]);
+
+    assertImportPrefix(findNode.simple('a.E'), findElement.prefix('a'));
+    var reference = findNode.functionReference('E<int>;');
+    assertFunctionReference(
+      reference,
+      findElement.importFind('package:test/a.dart').extension_('E'),
+      'dynamic',
+    );
+  }
+
   test_extensionGetter_extensionOverride() async {
     await assertErrorsInCode('''
 class A {}