Add test for ancestor/descendant type-equality constraints

tests/language-feature/generics/type-equality-ancestor-ordering.slang

@@ -0,0 +1,29 @@
+//DIAGNOSTIC_TEST:SIMPLE(diag=CHECK):
+
+struct Box<T>
+{
+    struct Nested<U> {}
+}
+
+// `Box<X>` is an ancestor of `Box<X>.Nested<X>`.
+void triggerA<X>() where Box<X> == Box<X>.Nested<X>
+{
+}
+
+void triggerB<X>() where Box<X>.Nested<X> == Box<X>
+{
+}
+
+void useIt()
+{
+    triggerA<int>();
+/*CHECK:
+    ^^^^^^^^ E38029
+    ^^^^^^^^ type argument 'Box<int>.Nested<int>' does not conform to the required interface 'Box<int>'
+*/
+    triggerB<int>();
+/*CHECK:
+    ^^^^^^^^ E38029
+    ^^^^^^^^ type argument 'Box<int>.Nested<int>' does not conform to the required interface 'Box<int>'
+*/
+}