gh-160719: Define _Py_STDC_VERSION in pyport.h

pyport.h uses _Py_STDC_VERSION to select _Py_thread_local, _Py_NULL and
_Py_TYPEOF, but the macro was defined in pymacro.h, which Python.h
includes after pyport.h. The C11 and C23 branches were therefore never
taken. Move the definition to pyport.h.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: skdas20

Include/pymacro.h

@@ -1,12 +1,7 @@
 #ifndef Py_PYMACRO_H
 #define Py_PYMACRO_H
 
-// _Py_STDC_VERSION: the version of the C standard.
-// gh-160208: The C++ standard allows __STDC_VERSION__ to be defined to an
-// arbitrary value, which means we can't use __STDC_VERSION__ on C++.
-#if !defined(__cplusplus) && defined(__STDC_VERSION__)
-#  define _Py_STDC_VERSION __STDC_VERSION__
-#endif
+// _Py_STDC_VERSION is defined in pyport.h, which is included before this file.
 
 // gh-91782: On FreeBSD 12, if the _POSIX_C_SOURCE and _XOPEN_SOURCE macros are
 // defined, <sys/cdefs.h> disables C11 support and <assert.h> does not define

Include/pyport.h

@@ -8,6 +8,15 @@
 #  error "Python's source code assumes C's unsigned char is an 8-bit type"
 #endif
 
+// _Py_STDC_VERSION: the version of the C standard.
+// gh-160208: The C++ standard allows __STDC_VERSION__ to be defined to an
+// arbitrary value, which means we can't use __STDC_VERSION__ on C++.
+// gh-160719: Defined here, rather than in pymacro.h, because this file uses it
+// and is included before pymacro.h.
+#if !defined(__cplusplus) && defined(__STDC_VERSION__)
+#  define _Py_STDC_VERSION __STDC_VERSION__
+#endif
+
 
 // Preprocessor check for a builtin preprocessor function. Always return 0
 // if __has_builtin() macro is not defined.

Misc/NEWS.d/next/C_API/2026-10-26-19-10-00.gh-issue-160719.F9F7fw.rst

@@ -0,0 +1,5 @@
+Fix the C11 and C23 code paths in :file:`pyport.h` never being used: the
+private ``_Py_STDC_VERSION`` macro was defined in :file:`pymacro.h`, which is
+included after :file:`pyport.h`, so ``_Py_thread_local``, ``_Py_NULL`` and
+``_Py_TYPEOF`` fell back to compiler extensions instead of ``_Thread_local``,
+``nullptr`` and ``typeof``.