analyzer: Report unused public members of private and unnamed extensions

Fixes #41592

Change-Id: I95b79724bfcfc5d48b8f6cca61c6a3a681de3662
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144353
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -323,13 +323,18 @@ class UnusedLocalElementsVerifier extends RecursiveAstVisitor {
     return _usedElements.elements.contains(element);
   }
 
-  bool _isUsedMember(Element element) {
+  bool _isUsedMember(ExecutableElement element) {
+    var enclosingElement = element.enclosingElement;
     if (element.isPublic) {
-      if (_isPrivateClassOrExtension(element.enclosingElement) &&
-          element is ExecutableElement &&
+      if (enclosingElement is ClassElement &&
+          enclosingElement.isPrivate &&
           element.isStatic) {
-        // Public static members of private classes, mixins, and extensions are
-        // inaccessible from outside the library in which they are declared.
+        // Public static members of private classes and mixins are inaccessible
+        // from outside the library in which they are declared.
+      } else if (enclosingElement is ExtensionElement &&
+          enclosingElement.isPrivate) {
+        // Public members of private extensions are inaccessible from outside
+        // the library in which they are declared.
       } else {
         return true;
       }

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

@@ -1414,7 +1414,7 @@ f() => E.a;
 
   test_thisAccessOnDynamic() async {
     await assertNoErrorsInCode('''
-extension on dynamic {
+extension E on dynamic {
   int get d => 3;
 
   void testDynamic() {
@@ -1427,7 +1427,7 @@ extension on dynamic {
 
   test_thisAccessOnFunction() async {
     await assertNoErrorsInCode('''
-extension on Function {
+extension E on Function {
   int get f => 4;
 
   void testFunction() {

pkg/analyzer/test/src/diagnostics/super_in_extension_test.dart

@@ -33,6 +33,7 @@ extension <T> on T {
   }
 }
 ''', [
+      error(HintCode.UNUSED_ELEMENT, 23, 1),
       error(CompileTimeErrorCode.SUPER_IN_EXTENSION, 33, 5),
     ]);
   }

pkg/analyzer/test/src/diagnostics/unused_element_test.dart

@@ -715,6 +715,17 @@ main() {
 ''');
   }
 
+  test_method_isUsed_privateExtension() async {
+    await assertNoErrorsInCode(r'''
+extension _A on String {
+  void m() {}
+}
+void main() {
+  "hello".m();
+}
+''');
+  }
+
   test_method_isUsed_staticInvocation() async {
     await assertNoErrorsInCode(r'''
 class A {
@@ -726,6 +737,17 @@ main() {
 ''');
   }
 
+  test_method_isUsed_unnamedExtension() async {
+    await assertNoErrorsInCode(r'''
+extension on String {
+  void m() {}
+}
+void main() {
+  "hello".m();
+}
+''');
+  }
+
   test_method_notUsed_hasSameNameAsUsed() async {
     await assertErrorsInCode(r'''
 class A {
@@ -750,6 +772,16 @@ class A {
     ]);
   }
 
+  test_method_notUsed_privateExtension() async {
+    await assertErrorsInCode(r'''
+extension _A on String {
+  void m() {}
+}
+''', [
+      error(HintCode.UNUSED_ELEMENT, 32, 1),
+    ]);
+  }
+
   test_method_notUsed_referenceFromItself() async {
     await assertErrorsInCode(r'''
 class A {
@@ -785,6 +817,16 @@ int g() => 7;
     ]);
   }
 
+  test_method_notUsed_unnamedExtension() async {
+    await assertErrorsInCode(r'''
+extension on String {
+  void m() {}
+}
+''', [
+      error(HintCode.UNUSED_ELEMENT, 29, 1),
+    ]);
+  }
+
   test_publicStaticMethod_privateClass_isUsed() async {
     await assertNoErrorsInCode(r'''
 class _A {

pkg/analyzer/test/src/diagnostics/use_of_void_result_test.dart

@@ -568,6 +568,7 @@ extension on void {
   }
 }
 ''', [
+      error(HintCode.UNUSED_ELEMENT, 22, 8),
       error(StaticWarningCode.USE_OF_VOID_RESULT, 96, 4),
     ]);
   }

pkg/analyzer/test/src/lint/linter/resolve_name_in_scope_test.dart

@@ -551,7 +551,9 @@ extension on A {
     this.foo();
   }
 }
-''');
+''', [
+      error(HintCode.UNUSED_ELEMENT, 53, 3),
+    ]);
     _checkMethodNone();
   }
 
@@ -566,7 +568,9 @@ extension on A {
     this.foo();
   }
 }
-''');
+''', [
+      error(HintCode.UNUSED_ELEMENT, 53, 3),
+    ]);
     _checkMethodRequested(findElement.parameter('foo'));
   }
 
@@ -583,7 +587,9 @@ extension on A {
 }
 
 var foo = 0;
-''');
+''', [
+      error(HintCode.UNUSED_ELEMENT, 53, 3),
+    ]);
     _checkMethodRequested(findElement.topGet('foo'));
   }
 
@@ -598,7 +604,9 @@ extension on A {
     this.foo();
   }
 }
-''');
+''', [
+      error(HintCode.UNUSED_ELEMENT, 53, 3),
+    ]);
     _checkMethodRequested(findElement.method('foo'));
   }