[tests] Add regression tests

Issue: #46867
Change-Id: I7e2912752ac8c48df233f27869599280e2c322fa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209853
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

tests/language/regress/regress46867_test.dart

@@ -0,0 +1,32 @@
+// Copyright (c) 2021, 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.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/46867
+
+import 'package:expect/expect.dart';
+
+class Interface {
+  covariant Object x = 'from Interface';
+}
+
+mixin Mixin implements Interface {}
+
+class BaseClass {
+  static var getterCallCount = 0;
+  static var setterCallCount = 0;
+  Object get x => getterCallCount++;
+  set x(Object value) => setterCallCount++;
+}
+
+class SubClass extends BaseClass with Mixin {}
+
+void main() {
+  Expect.equals(0, BaseClass.getterCallCount);
+  SubClass().x;
+  Expect.equals(1, BaseClass.getterCallCount);
+
+  Expect.equals(0, BaseClass.setterCallCount);
+  SubClass().x = 42;
+  Expect.equals(1, BaseClass.setterCallCount);
+}

tests/language_2/regress/regress46867_test.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2021, 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.
+
+// @dart = 2.9
+
+// Regression test for https://github.com/dart-lang/sdk/issues/46867
+
+import 'package:expect/expect.dart';
+
+class Interface {
+  covariant Object x = 'from Interface';
+}
+
+mixin Mixin implements Interface {}
+
+class BaseClass {
+  static var getterCallCount = 0;
+  static var setterCallCount = 0;
+  Object get x => getterCallCount++;
+  set x(Object value) => setterCallCount++;
+}
+
+class SubClass extends BaseClass with Mixin {}
+
+void main() {
+  Expect.equals(0, BaseClass.getterCallCount);
+  SubClass().x;
+  Expect.equals(1, BaseClass.getterCallCount);
+
+  Expect.equals(0, BaseClass.setterCallCount);
+  SubClass().x = 42;
+  Expect.equals(1, BaseClass.setterCallCount);
+}