When the Python C API is used with GCC and clang, I'm pretty sure that __extension__ can be used to get an anonymous union, without emitting new compiler warnings or errors, even when -pedantic option is used. For now, I prefer to skip this code path to focus on C11 vs C99 code path, so I commented the code (you can uncomment it to test).

I don't know if MSC has pragma to turn off the compiler warning on "Compiler Warning (level 4) C4201": "nonstandard extension used: nameless struct/union".

For now, my PR implements Py_INCREF() and Py_DECREF() as opaque function calls on C99 and newer. Maybe later, this code path can still be optimized to inline the code. But a solution without anonymous union should be found for that.

To test this PR, I added the following code to Modules/_testcapimodule.c and Programs/_testembed.c:

#ifdef _PYOBJECT_REFCNT_ANON_UNION
    printf("PyObject.ob_refcnt is an union\n");
#else
    printf("PyObject.ob_refcnt type is Py_ssize_t\n");
#endif

And I modified Makefile.pre.in to build Programs/_testembed.c with -std=c99: in this case, the union is not used, whereas _testcapi is built with the union (it's built with -std=c11).

This change can have a significant negative impact on performance, so I abandon it and instead try to get rid of the compiler warning: PR #107232.