libstdc++: add a constexpr macro for C++26

Following the precedent of _GLIBCXX20_CONSTEXPR.

It will be used to decorate some functions which have been made
constexpr in C++26 (for instance P2562R1, and maybe P3508R0, P3369R0,
...).

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX26_CONSTEXPR): New macro.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 07f75ea66595051733a0c22f84a089a524541d60..6a73507d0748fdcb2fb950d6b45bb54435a78d4b 100644 (file)
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -222,6 +222,14 @@
 # endif
 #endif
 
+#ifndef _GLIBCXX26_CONSTEXPR
+# if __cplusplus >= 202400L
+#  define _GLIBCXX26_CONSTEXPR constexpr
+# else
+#  define _GLIBCXX26_CONSTEXPR
+# endif
+#endif
+
 #ifndef _GLIBCXX17_INLINE
 # if __cplusplus >= 201703L
 #  define _GLIBCXX17_INLINE inline