Add (passing) test for type parameters in exhaustiveness checking.

Change-Id: I2b2dc6a586d66c93e918b077bd008c3683986d18
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/301300
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>

tests/language/patterns/exhaustiveness/type_test_type_parameter_test.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=patterns
+
+// Test that using a type parameter as a type test in a pattern doesn't
+// incorrectly use the bound for exhaustiveness.
+//
+// The bound isn't a correct approximation because the type argument could be
+// a subtype of the bound (including `Never`), which will cause the type test
+// to match fewer values than the bound.
+
+void test<T extends int>(num value) {
+  // Object pattern.
+  var result = switch (value) {
+    //         ^^^^^^
+    // [analyzer] COMPILE_TIME_ERROR.NON_EXHAUSTIVE_SWITCH_EXPRESSION
+    //                 ^
+    // [cfe] The type 'num' is not exhaustively matched by the switch cases since it doesn't match 'int()'.
+    T() => 'T',
+    double() => 'double'
+  };
+
+  // Variable type.
+  result = switch (value) {
+    //     ^^^^^^
+    // [analyzer] COMPILE_TIME_ERROR.NON_EXHAUSTIVE_SWITCH_EXPRESSION
+    //             ^
+    // [cfe] The type 'num' is not exhaustively matched by the switch cases since it doesn't match 'int()'.
+    T _ => 'T',
+    double _ => 'double'
+  };
+}