[gcc r16-2772] c++: add another testcase [PR121351]

[Patrick Palka via Gcc-cvs]

https://gcc.gnu.org/g:b9da8541c4acfc0a65f574bace6ca160bf70cc01

commit r16-2772-gb9da8541c4acfc0a65f574bace6ca160bf70cc01
Author: Patrick Palka <ppa...@redhat.com>
Date:   Mon Aug 4 17:09:28 2025 -0400

    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.

Diff:
---
 gcc/testsuite/g++.dg/cpp2a/concepts-using6.C | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-using6.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-using6.C
new file mode 100644
index 000000000000..a40519a30dae
--- /dev/null
+++ b/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
+}