error code for ambiguous extension method accesses

Change-Id: I97b0bc8ba32d1cb0ee3b361525f4c161845f1d01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/109463
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/error/error.dart

@@ -66,6 +66,7 @@ const List<ErrorCode> errorCodeValues = const [
   CompileTimeErrorCode.ABSTRACT_SUPER_MEMBER_REFERENCE,
   CompileTimeErrorCode.ACCESS_PRIVATE_ENUM_FIELD,
   CompileTimeErrorCode.AMBIGUOUS_EXPORT,
+  CompileTimeErrorCode.AMBIGUOUS_EXTENSION_METHOD_ACCESS,
   CompileTimeErrorCode.AMBIGUOUS_SET_OR_MAP_LITERAL_BOTH,
   CompileTimeErrorCode.AMBIGUOUS_SET_OR_MAP_LITERAL_EITHER,
   CompileTimeErrorCode.ANNOTATION_WITH_NON_CLASS,

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

@@ -158,13 +158,12 @@ class MethodInvocationResolver {
       var t1 = e1.extendedType;
       var t2 = e2.extendedType;
 
-      // todo(pq): broaden platform beyond dart:core
-      if (t2.element.library.isDartCore) {
+      if (t2.element.library.isInSdk) {
         //  1. T2 is declared in a platform library, and T1 is not
-        if (!t1.element.library.isDartCore) {
+        if (!t1.element.library.isInSdk) {
           return -1;
         }
-      } else if (t1.element.library.isDartCore) {
+      } else if (t1.element.library.isInSdk) {
         return 1;
       }
 
@@ -504,7 +503,17 @@ class MethodInvocationResolver {
       ExtensionElement extension =
           _chooseMostSpecificExtension(extensions, receiverType);
       if (extension == null) {
-        // todo(pq): report an error
+        _setDynamicResolution(node);
+        _resolver.errorReporter.reportErrorForNode(
+          CompileTimeErrorCode.AMBIGUOUS_EXTENSION_METHOD_ACCESS,
+          nameNode,
+          [
+            name,
+            extensions[0].name,
+            extensions[1].name,
+          ],
+        );
+        return;
       } else {
         Element member = extension.getMethod(name) ??
             extension.getGetter(name) ??

pkg/analyzer/lib/src/error/codes.dart

@@ -142,6 +142,24 @@ class CompileTimeErrorCode extends ErrorCode {
           correction: "Try removing the export of one of the libraries, or "
               "explicitly hiding the name in one of the export directives.");
 
+  /**
+   * It is a compile time error if there are two applicable extensions defining
+   * the same member and neither is more specific than the other.
+   *
+   * Parameters:
+   * 0: the name of the member
+   * 1: the name of the first declaring extension
+   * 2: the name of the second declaring extension
+   */
+  static const CompileTimeErrorCode AMBIGUOUS_EXTENSION_METHOD_ACCESS =
+      const CompileTimeErrorCode(
+          'AMBIGUOUS_EXTENSION_METHOD_ACCESS',
+          "A member named '{0}' is defined in extensions '{1}' and '{2}' and "
+              "neither is more specific.",
+          correction:
+              "Try using an extension override to specify the extension "
+              "you want to to be chosen.");
+
   /**
    * No parameters.
    */

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

@@ -60,10 +60,6 @@ f() {
     expect(invocation.methodName.staticElement.library.isDartCore, isFalse);
   }
 
-  test_multi_match_ambiguous() async {
-    // todo(pq): implement
-  }
-
   test_multipleExtensions() async {
     await assertNoErrorsInCode('''
 class A {}
@@ -87,6 +83,27 @@ f() {
 ''', [StaticTypeWarningCode.UNDEFINED_METHOD]);
   }
 
+  test_noMostSpecificExtension() async {
+    await assertErrorsInCode('''
+class A { }
+
+extension A1_Ext on A {
+  void a() { }
+}
+
+extension A2_Ext on A {
+  void a() { }
+}
+
+f() {
+  A a = A();
+  a.a();
+}
+''', [
+      error(CompileTimeErrorCode.AMBIGUOUS_EXTENSION_METHOD_ACCESS, 120, 1),
+    ]);
+  }
+
   test_one_match() async {
     await assertNoErrorsInCode('''
 class B { }