+ extension type target kind (and @optionalTypeArgs support)

Adds a new TargetKind for extension types and adds it to `@optionalTypeArgs`.

Change-Id: I7254c1299c296451b9adc5d1f8eceb50af66f039
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324769
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/analysis_server/test/mock_packages/meta/lib/meta.dart

@@ -518,6 +518,7 @@ class _NonVirtual {
 @Target({
   TargetKind.classType,
   TargetKind.extension,
+  TargetKind.extensionType,
   TargetKind.function,
   TargetKind.method,
   TargetKind.mixinType,

pkg/analysis_server/test/mock_packages/meta/lib/meta_meta.dart

@@ -39,6 +39,9 @@ class TargetKind {
   /// Indicates that an annotation is valid on any extension declaration.
   static const extension = TargetKind._('extensions', 'extension');
 
+  /// Indicates that an annotation is valid on any extension type declaration.
+  static const extensionType = TargetKind._('extension types', 'extensionType');
+
   /// Indicates that an annotation is valid on any field declaration, both
   /// instance and static fields, whether it's in a class, mixin or extension.
   static const field = TargetKind._('fields', 'field');
@@ -91,6 +94,7 @@ class TargetKind {
     classType,
     enumType,
     extension,
+    extensionType,
     field,
     function,
     library,

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

@@ -554,6 +554,8 @@ class AnnotationVerifier {
     } else if (target is EnumDeclaration) {
       return kinds.contains(TargetKind.enumType) ||
           kinds.contains(TargetKind.type);
+    } else if (target is ExtensionTypeDeclaration) {
+      return kinds.contains(TargetKind.extensionType);
     } else if (target is ExtensionDeclaration) {
       return kinds.contains(TargetKind.extension);
     } else if (target is FieldDeclaration) {

pkg/analyzer/lib/src/test_utilities/mock_packages.dart

@@ -238,6 +238,7 @@ class _NonVirtual {
 @Target({
   TargetKind.classType,
   TargetKind.extension,
+  TargetKind.extensionType,
   TargetKind.function,
   TargetKind.method,
   TargetKind.mixinType,
@@ -284,6 +285,7 @@ class TargetKind {
   static const classType = TargetKind._('classes', 'classType');
   static const enumType = TargetKind._('enums', 'enumType');
   static const extension = TargetKind._('extensions', 'extension');
+  static const extensionType = TargetKind._('extension types', 'extensionType');
   static const field = TargetKind._('fields', 'field');
   static const function = TargetKind._('top-level functions', 'function');
   static const library = TargetKind._('libraries', 'library');
@@ -302,6 +304,7 @@ class TargetKind {
     classType,
     enumType,
     extension,
+    extensionType,
     field,
     function,
     library,

pkg/linter/test/rules/always_specify_types_test.dart

@@ -14,9 +14,38 @@ main() {
 
 @reflectiveTest
 class AlwaysSpecifyTypesTest extends LintRuleTest {
+  @override
+  bool get addMetaPackageDep => true;
+
   @override
   String get lintRule => 'always_specify_types';
 
+  test_extensionType_optionalTypeArgs() async {
+    await assertNoDiagnostics(r'''
+import 'package:meta/meta.dart';
+
+@optionalTypeArgs
+extension type E<T>(int i) { }
+
+void f() {
+  E e = E(1);
+}
+''');
+  }
+
+  test_extensionType_typeArgs() async {
+    await assertDiagnostics(r'''
+extension type E<T>(int i) { }
+
+void f() {
+  E e = E(1);
+}
+''', [
+      lint(45, 1),
+      lint(51, 1),
+    ]);
+  }
+
   test_listPattern_destructured() async {
     await assertDiagnostics(r'''
 f() {

pkg/meta/lib/meta.dart

@@ -518,6 +518,7 @@ class _NonVirtual {
 @Target({
   TargetKind.classType,
   TargetKind.extension,
+  TargetKind.extensionType,
   TargetKind.function,
   TargetKind.method,
   TargetKind.mixinType,

pkg/meta/lib/meta_meta.dart

@@ -39,6 +39,9 @@ class TargetKind {
   /// Indicates that an annotation is valid on any extension declaration.
   static const extension = TargetKind._('extensions', 'extension');
 
+  /// Indicates that an annotation is valid on any extension type declaration.
+  static const extensionType = TargetKind._('extension types', 'extensionType');
+
   /// Indicates that an annotation is valid on any field declaration, both
   /// instance and static fields, whether it's in a class, mixin or extension.
   static const field = TargetKind._('fields', 'field');
@@ -91,6 +94,7 @@ class TargetKind {
     classType,
     enumType,
     extension,
+    extensionType,
     field,
     function,
     library,