[gcc r16-1501] libstdc++: add a workaround for format_kind<optional<T>> [PR120644]

[Giuseppe D'Angelo]

https://gcc.gnu.org/g:f6f4efdf397269793167fb1303967e7ad0e467a7

commit r16-1501-gf6f4efdf397269793167fb1303967e7ad0e467a7
Author: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Date:   Fri Jun 13 11:38:23 2025 +0200

    libstdc++: add a workaround for format_kind<optional<T>> [PR120644]
    
    The specialization of format_kind for optional is causing a problem when
    optional is imported and included. The comments on the PR strongly
    suggest that this is a frontend bug; this commit just works around the
    issue by specifying the type of format_kind<optional<T>> to be
    `range_format`, rather than leaving the compiler deduce it via `auto`.
    
            PR c++/120644
    
    libstdc++-v3/ChangeLog:
    
            * include/std/optional (format_kind): Do not use `auto`.

Diff:
---
 libstdc++-v3/include/std/optional | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 2ae71f11e213..cc7af5bbd7d2 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1815,7 +1815,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       ranges::enable_view<optional<_Tp>> = true;
 
   template<typename _Tp>
-    inline constexpr auto
+    inline constexpr range_format
       format_kind<optional<_Tp>> = range_format::disabled;
 #endif // __cpp_lib_optional_range_support