Migration: fix crashes with extension Foo<T extends ...> on T

Change-Id: I3a8f1be4f52a6a49bb57ab032fef668482f5acf6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151002
Reviewed-by: Mike Fairhurst <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -236,7 +236,8 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       } else {
         assert(enclosingElement is ExtensionElement);
         final extensionElement = enclosingElement as ExtensionElement;
-        final extendedType = extensionElement.extendedType;
+        final extendedType =
+            _typeSystem.resolveToBound(extensionElement.extendedType);
         if (extendedType is InterfaceType) {
           if (extensionElement.typeParameters.isNotEmpty) {
             substitution = _decoratedClassHierarchy
@@ -743,6 +744,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
 
   DecoratedType visitExtensionDeclaration(ExtensionDeclaration node) {
     visitClassOrMixinOrExtensionDeclaration(node);
+    _dispatch(node.typeParameters);
     _dispatch(node.extendedType);
     return null;
   }
@@ -2917,6 +2919,8 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
                     NullabilityNode.forInferredType(
                         target.typeArgument(index++))))
                 .toList());
+      } else if (type is TypeParameterType) {
+        return DecoratedType(type, NullabilityNode.forInferredType(target));
       } else {
         _unimplemented(node, 'extension of $type (${type.runtimeType}');
       }

pkg/nnbd_migration/test/api_test.dart

@@ -2082,6 +2082,48 @@ void g() => f(null);
     await _checkSingleFileChanges(content, expected);
   }
 
+  Future<void> test_extension_on_type_param_implementation() async {
+    var content = '''
+abstract class C {
+  C _clone();
+}
+extension Cloner<T extends C> on T {
+  T clone() => _clone() as T;
+}
+''';
+    var expected = '''
+abstract class C {
+  C _clone();
+}
+extension Cloner<T extends C> on T {
+  T clone() => _clone() as T;
+}
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
+  Future<void> test_extension_on_type_param_usage() async {
+    var content = '''
+abstract class C {
+  C _clone();
+}
+extension Cloner<T extends C> on T {
+  T clone() => throw Exception();
+}
+C f(C c) => c.clone();
+''';
+    var expected = '''
+abstract class C {
+  C _clone();
+}
+extension Cloner<T extends C> on T {
+  T clone() => throw Exception();
+}
+C f(C c) => c.clone();
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
   Future<void> test_field_final_uninitalized_used() async {
     var content = '''
 class C {