#3057. Add "diamond" inheritance case to promotion via assignment tests (#3179)

Author: sgrekhov

TypeSystem/flow-analysis/promotion_via_assignment_A06_t02.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2025, 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.
+
+/// @assertion We say that a variable `x` is promotable via assignment of an
+/// expression of type `T` given variable model `VM` if
+/// - `VM = VariableModel(declared, promoted, tested, assigned, unassigned,
+///     captured)`
+/// - and captured is false
+/// - and `S` is the current type of `x` in `VM`
+/// - and `T <: S` and not `S <: T`
+/// - and `T` is a type of interest for `x` in `tested`
+///
+/// @description Checks static type of a variable after assignment with a
+/// supertype of the declared type as type of interest. Test the "diamond"
+/// inheritance.
+/// @author sgrekhov22@gmail.com
+/// @issue 60622
+
+import '../../Utils/static_type_helper.dart';
+
+class A {}
+class B extends A {}
+class C extends A {}
+class D implements B, C {}
+
+void test(Object a, D d){
+  a as A;
+  if (a is B || a is C) {}
+  a = d; // Not promoted: Neither `B` nor `C` is minimal in `{A, B, C}`.
+  a.expectStaticType<Exactly<A>>();
+}
+
+main() {
+  test(A(), D());
+}