[gcc r15-8655] c++: add fixed test [PR119378]

[Patrick Palka]

https://gcc.gnu.org/g:6ed38b98456725f042e75fde7da16c60e22c08d8

commit r15-8655-g6ed38b98456725f042e75fde7da16c60e22c08d8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Mar 21 15:23:49 2025 -0400

    c++: add fixed test [PR119378]
    
    Fixed by r15-123 (specifically the change to set processing_template_decl
    when tsubsting UNBOUND_CLASS_TEMPLATE).
    
            PR c++/119378
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/friend85.C: New test.

Diff:
---
 gcc/testsuite/g++.dg/template/friend85.C | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/gcc/testsuite/g++.dg/template/friend85.C b/gcc/testsuite/g++.dg/template/friend85.C
new file mode 100644
index 000000000000..5cf839111931
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/friend85.C
@@ -0,0 +1,16 @@
+// PR c++/119378
+
+template<int N>
+struct A {
+  template<class T>
+  struct B;
+};
+
+template<class U>
+struct C {
+  template<int N>
+  template<class T>
+  friend class A<N>::B;
+};
+
+template struct C<int>;