[gcc r15-6696] libstdc++: add missing to_underlying to module std [PR106852]

[Jonathan Wakely]

https://gcc.gnu.org/g:653a44e8613edf6c306687d17cf3039536f1bf12

commit r15-6696-g653a44e8613edf6c306687d17cf3039536f1bf12
Author: Nicolas Werner <nicolas.werner@hotmail.de>
Date:   Tue Jan 7 15:26:54 2025 +0000

    libstdc++: add missing to_underlying to module std [PR106852]
    
    std::to_underlying was missing from the std module introduced in
    r15-5366-g7db55c0ba1baaf. This patch adds the missing export for this
    utility.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/106852
            * src/c++23/std.cc.in (to_underlying): Add.
    
    Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

Diff:
---
 libstdc++-v3/src/c++23/std.cc.in | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index dcba343426c6..c0b7e1dc7271 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -3173,6 +3173,9 @@ export namespace std
   using std::make_pair;
   using std::piecewise_construct;
   using std::piecewise_construct_t;
+#if __cpp_lib_to_underlying
+  using std::to_underlying;
+#endif
   using std::tuple_element;
   using std::tuple_size;
 #pragma GCC diagnostic push