[gcc r16-8950] libstdc++: Fix recent std::flat_map test with COW std::string

[Patrick Palka]

https://gcc.gnu.org/g:92f550b49b3ab4c9b7b06222d71b2f2c10843657

commit r16-8950-g92f550b49b3ab4c9b7b06222d71b2f2c10843657
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 21 18:35:55 2026 -0400

    libstdc++: Fix recent std::flat_map test with COW std::string
    
    ... which is not constexpr, so just disable constant evaluation of the
    test in this case.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/23_containers/flat_map/1.cc (test_constexpr):
            If std::string isn't constexpr don't run test10.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    (cherry picked from commit ea57201777f8cb58766953f87880bcf89244411b)

Diff:
---
 libstdc++-v3/testsuite/23_containers/flat_map/1.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
index 096b9eeac3e4..1cb088f511eb 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -330,7 +330,9 @@ test_constexpr()
   test07();
   test08();
   test09();
+#if __cpp_lib_constexpr_string >= 201907L
   test10();
+#endif
   return true;
 }