Extension type. Issue 53866. Allow the representation type to be the subtype of implemented extension type.

Bug: #53866
Change-Id: Ic3ac0556695c8ffbb8a4545777ea07a8b52c1e76
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332363
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

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

@@ -1785,11 +1785,15 @@ class ResolutionVisitor extends RecursiveAstVisitor<void> {
       return;
     }
 
+    final declaredRepresentation = declaredElement.representation.type;
+    if (typeSystem.isSubtypeOf(declaredRepresentation, type)) {
+      return;
+    }
+
     // When `type` is an extension type.
     if (type is InterfaceTypeImpl) {
       final implementedRepresentation = type.representationType;
       if (implementedRepresentation != null) {
-        final declaredRepresentation = declaredElement.representation.type;
         if (!typeSystem.isSubtypeOf(
           declaredRepresentation,
           implementedRepresentation,
@@ -1810,14 +1814,11 @@ class ResolutionVisitor extends RecursiveAstVisitor<void> {
       }
     }
 
-    final declaredRepresentation = declaredElement.representation.type;
-    if (!typeSystem.isSubtypeOf(declaredRepresentation, type)) {
-      _errorReporter.reportErrorForNode(
-        CompileTimeErrorCode.EXTENSION_TYPE_IMPLEMENTS_NOT_SUPERTYPE,
-        node,
-        [type, declaredRepresentation],
-      );
-    }
+    _errorReporter.reportErrorForNode(
+      CompileTimeErrorCode.EXTENSION_TYPE_IMPLEMENTS_NOT_SUPERTYPE,
+      node,
+      [type, declaredRepresentation],
+    );
   }
 
   void _visitIf(IfElementOrStatementImpl node) {

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

@@ -44,6 +44,14 @@ extension type B(S3 it) implements A {}
 class S1 {}
 class S2 extends S1 {}
 class S3 extends S2 {}
+''');
+  }
+
+  test_supertype3() async {
+    await assertNoErrorsInCode('''
+extension type V1(num _) {}
+extension type V2(int _) implements V1 {}
+extension type ET(V2 id) implements V1 {}
 ''');
   }
 }