c++: add another testcase [PR121351]

Here's a previously accepted testcase that is now ambiguous after
r16-2771-gb9f1cc4e119da, since the uninstantiated constraints are
equivalent but the partially instantiated constraints aren't, so
the two member functions no longer correspond.

	PR c++/121351

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-using6.C: New test.

gcc/testsuite/g++.dg/cpp2a/concepts-using6.C

@@ -0,0 +1,20 @@
+// PR c++/121351
+// { dg-do compile { target c++20 } }
+
+template<class T> concept C = true;
+
+template<class T>
+struct A {
+  template<class U> void f(U) requires C<T>; // #1
+};
+
+template<class T>
+struct B : A<T*> {
+  using A<T*>::f;
+  template<class U> void f(U) requires C<T>; // #2
+};
+
+int main() {
+  B<int> b;
+  b.f(42); // { dg-error "ambiguous" } #1 and #2 don't correspond
+}