fix #26965, allow promotion from type param upper bound in strong mode

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.src.task.strong.checker_test;
 
 import '../../../reflective_tests.dart';
 import 'strong_test_helper.dart';
 
 void main() {
@@ skipping 3404 matching lines @@
 g() {
   Object x;
   if (x is int) {
     int y = x;
     String z = /*error:INVALID_ASSIGNMENT*/x;
   }
 }
 ''');
   }
 
+  void test_typePromotionFromTypeParameter() {
+    // Regression test for https://github.com/dart-lang/sdk/issues/26965
+    checkFile(r'''
+void f/*<T>*/(/*=T*/ object) {
+  if (object is String) print(object.substring(1));
+}
+void g/*<T extends num>*/(/*=T*/ object) {
+  if (object is int) print(object.isEven);
+  if (object is String) print(/*info:DYNAMIC_INVOKE*/object.substring(1));
+}
+class Clonable<T> {}
+class SubClonable<T> extends Clonable<T> {
+  T m(T t) => t;
+}
+void h/*<T extends Clonable<T>>*/(/*=T*/ object) {
+  if (/*info:NON_GROUND_TYPE_CHECK_INFO*/object is SubClonable/*<T>*/) {
+    // Note we need to cast back to T, because promotion lost that type info.
+    print(object.m(object as dynamic/*=T*/));
+  }
+}
+''');
+  }
+
   void test_typeSubtyping_assigningClass() {
     checkFile('''
 class A {}
 class B extends A {}
 
 void main() {
    dynamic y;
    Object o;
    int i = 0;
    double d = 0.0;
@@ skipping 190 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }