[gcc r15-3133] libstdc++: Optimize __try_use_facet for const types

[Jonathan Wakely]

https://gcc.gnu.org/g:952e67c0d131300f263d729a0fe07bc3655dad27

commit r15-3133-g952e67c0d131300f263d729a0fe07bc3655dad27
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 22 16:49:31 2024 +0100

    libstdc++: Optimize __try_use_facet for const types
    
    LWG 436 confirmed that const-qualified types are valid arguments for
    Facet template parameters (but volatile-qualified types are not). Use the
    fast path in std::use_facet and std::has_facet for const T as well as T.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/locale_classes.tcc (__try_use_facet): Also avoid
            dynamic_cast for const-qualified facet types.

Diff:
---
 libstdc++-v3/include/bits/locale_classes.tcc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/locale_classes.tcc b/libstdc++-v3/include/bits/locale_classes.tcc
index c79574e58de8..d5ef1911057b 100644
--- a/libstdc++-v3/include/bits/locale_classes.tcc
+++ b/libstdc++-v3/include/bits/locale_classes.tcc
@@ -110,7 +110,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // We know these standard facets are always installed in every locale
       // so dynamic_cast always succeeds, just use static_cast instead.
 #define _GLIBCXX_STD_FACET(...) \
-      if _GLIBCXX_CONSTEXPR (__is_same(_Facet, __VA_ARGS__)) \
+      if _GLIBCXX_CONSTEXPR (__is_same(const _Facet, const __VA_ARGS__)) \
 	return static_cast<const _Facet*>(__facets[__i])
 
       _GLIBCXX_STD_FACET(ctype<char>);