[gcc r16-5915] libstdc++: Introduce _GLIBCXX_EXPLICIT_THIS_PARAMETER internal FTM

[Patrick Palka]

https://gcc.gnu.org/g:101f96847629850edbdfceb11b363770be4fd69d

commit r16-5915-g101f96847629850edbdfceb11b363770be4fd69d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 5 12:14:18 2025 -0500

    libstdc++: Introduce _GLIBCXX_EXPLICIT_THIS_PARAMETER internal FTM
    
    This FTM is like __cpp_explicit_this_parameter but is also defined
    in earlier C++ modes if deducing this is supported as an extension
    by the compiler.  Currently only GCC supports this, Clang doesn't.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/c++config (_GLIBCXX_EXPLICIT_THIS_PARAMETER):
            New.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/c++config | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index eec3a4a499dd..e6d8f186d0d4 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -927,6 +927,13 @@ namespace __gnu_cxx
 # define _GLIBCXX_USE_BUILTIN_TRAIT(BT) 0
 #endif
 
+// Whether deducing this is usable either officially, if in C++23 mode, or
+// as an extension (Clang doesn't support the latter).
+#if __cpp_explicit_this_parameter \
+  || (__cplusplus >= 201103L && __GNUC__ >= 14 && !defined(_GLIBCXX_CLANG))
+# define _GLIBCXX_EXPLICIT_THIS_PARAMETER 202110L
+#endif
+
 // Mark code that should be ignored by the compiler, but seen by Doxygen.
 #define _GLIBCXX_DOXYGEN_ONLY(X)