[gcc r13-2826] libstdc++: Add test for type traits not having friend access

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:5924c7d584640665db174c7545ae6c2b784af27c

commit r13-2826-g5924c7d584640665db174c7545ae6c2b784af27c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 23 22:04:24 2022 +0100

    libstdc++: Add test for type traits not having friend access
    
    This ensures that the std::is_assignable and std::is_assignable_v
    traits are evaluated "in a context unrelated" to the argument types.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/20_util/is_assignable/requirements/access.cc:
            New test.

Diff:
---
 .../20_util/is_assignable/requirements/access.cc   | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/libstdc++-v3/testsuite/20_util/is_assignable/requirements/access.cc b/libstdc++-v3/testsuite/20_util/is_assignable/requirements/access.cc
new file mode 100644
index 00000000000..a96fba654cd
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/is_assignable/requirements/access.cc
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++11 } }
+
+#include <type_traits>
+
+class S {
+  operator int();
+  friend void g(); // #1
+};
+
+void
+g()
+{
+  int i = 0;
+  S s;
+  i = s; // this works, because we're inside a friend.
+
+  // But the traits are evaluated in "a context unrelated to either type".
+  static_assert( ! std::is_assignable<int&, S>::value, "unfriendly");
+#if __cplusplus >= 201703L
+  static_assert( ! std::is_assignable_v<int&, S>, "unfriendly");
+#endif
+}