[gcc r16-2294] libstdc++: Use basic_string_view for std::bitset deduced types

[Jonathan Wakely]

https://gcc.gnu.org/g:7c3081960922d9bd3ea81a9f8ca6e00c23170a2b

commit r16-2294-g7c3081960922d9bd3ea81a9f8ca6e00c23170a2b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Jul 12 13:12:09 2025 +0100

    libstdc++: Use basic_string_view for std::bitset deduced types
    
    This change was part of by P2697R1 (Interfacing bitset with string_view)
    and should be slightly cheaper to instantiate.
    
    We should consider using basic_string_view for C++17, C++20, and C++23
    as well. This patch just conservatively changes it for C++26 to match
    the working draft. It's conceivable that a program-defined
    specialization of basic_string<_CharT> or basic_string_view<_CharT> will
    observe a difference and be affected by this change.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/bitset (__bitset::__string) [__cpp_lib_bitset]:
            Change alias to refer to basic_string_view instead.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/std/bitset | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset
index 1c1e1670c33f..93a03f6b5d72 100644
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -720,7 +720,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
   namespace __bitset
   {
-#if _GLIBCXX_HOSTED
+#ifdef __cpp_lib_bitset // ...construct from string_view
+    template<typename _CharT>
+      using __string = std::basic_string_view<_CharT>;
+#elif _GLIBCXX_HOSTED
     template<typename _CharT>
       using __string = std::basic_string<_CharT>;
 #else