After upgrading python3-devel from 0:3.15.1-1.fc46.x86_64 to 0:3.15.2-1.fc46.x86_64, building Python extensions with -Werror=undef fails:

$ echo '#include <Python.h>' | gcc -x c -Werror=undef -I/usr/include/python3.15 -c -
In file included from /usr/include/python3.15/Python.h:68,
                 from <stdin>:1:
/usr/include/python3.15/pyport.h:494:9: error: ‘_Py_STDC_VERSION’ is not defined, evaluates to ‘0’ [-Werror=undef]
  494 | #  elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
      |         ^~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

The cause is a missing check for _Py_STDC_VERSION definition before comparing _Py_STDC_VERSION value in /usr/include/python3.15/pyport.h:

It reads:

  #  elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)

and it should be:

  #  elif defined(_Py_STDC_VERSION) && _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)

Compare to changes made in /usr/include/python3.15/pyport.h between the two version:

@@ -50,7 +50,7 @@
 // to prevent C++ compiler warnings. On C23 and newer and on C++11 and newer,
 // _Py_NULL is defined as nullptr.
 #if !defined(_MSC_VER) && \
-    ((defined (__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) \
+    ((defined (_Py_STDC_VERSION) && _Py_STDC_VERSION >= 202311L) \
         || (defined(__cplusplus) && __cplusplus >= 201103))
 #  define _Py_NULL nullptr
 #else
@@ -491,7 +491,7 @@
 #ifdef WITH_THREAD
 #  ifdef thread_local
 #    define _Py_thread_local thread_local
-#  elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+#  elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
 #    define _Py_thread_local _Thread_local
 #  elif defined(_MSC_VER)  /* AKA NT_THREADS */
 #    define _Py_thread_local __declspec(thread)
@@ -543,7 +543,7 @@
 //
 // gh-157649: Do not use decltype() on C++, since it produces invalid code in
 // Py_CLEAR()/Py_SETREF().
-#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#if defined (_Py_STDC_VERSION) && _Py_STDC_VERSION >= 202311L
 #  define _Py_TYPEOF(expr) typeof(expr)
 #elif (defined(__GNUC__) || defined(__clang__) \
        || (defined(_MSC_VER) && _MSC_VER >= 1939 && !defined(__cplusplus)))