[Bug libstdc++/125450] [16/17 Regression] std::optional breaks in C++26 freestanding mode

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125450

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We need this:

--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -2286,9 +2286,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     ranges::enable_borrowed_range<optional<_Tp&>> = true;
 #endif

+#if _GLIBCXX_HOSTED
   template<typename _Tp>
     inline constexpr range_format
       format_kind<optional<_Tp>> = range_format::disabled;
+#endif
 #endif // __cpp_lib_optional_range_support

 #undef _GLIBCXX_USE_CONSTRAINTS_FOR_OPTIONAL