gh-105059: Fix MSCV compiler warning on PyObject union

[vstinner]

Use pragma to ignore the MSCV compiler warning on the PyObject nameless union.

• Issue: object.h uses an anonymous union in a struct (older C incompatible) #105059

[vstinner]

@zooba: Would you be able to check if this change fix the compiler warning for you? These warnings: #105059 (comment)

[vstinner]

I managed to reproduce the issue with this repro.c file:

#include <Python.h>
int main () {
    return 0;
}

I open the "Developer Command Prompt", go Python source code directory, and type cl /c /I Include /I PC /W4 repro.c.

Output on the main branch:

Microsoft (R) C/C++ Optimizing Compiler Version 19.36.32537 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.c
C:\victor\python\main\Include\object.h(178): warning C4201: nonstandard extension used: nameless struct/union
C:\victor\python\main\Include\cpython/unicodeobject.h(199): warning C4100: '_unused_op': unreferenced formal parameter
C:\victor\python\main\Include\cpython/unicodeobject.h(387): warning C4100: '_unused_op': unreferenced formal parameter

=> The warning C4201: nonstandard extension used: nameless struct/union warning is there.

Output with my PR:

Microsoft (R) C/C++ Optimizing Compiler Version 19.36.32537 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.c
C:\victor\python\main\Include\cpython/unicodeobject.h(199): warning C4100: '_unused_op': unreferenced formal parameter
C:\victor\python\main\Include\cpython/unicodeobject.h(387): warning C4100: '_unused_op': unreferenced formal parameter

=> The waning is gone!

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-107248 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Gentoo Installed with X 3.x has failed when building commit 1c8fe9b.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/464/builds/5113) and take a look at the build logs.
4. Check if the failure is related to this commit (1c8fe9b) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/464/builds/5113

Failed tests:

• test.test_asyncio.test_subprocess

Failed subtests:

• test_subprocess_consistent_callbacks - test.test_asyncio.test_subprocess.SubprocessThreadedWatcherTests.test_subprocess_consistent_callbacks

Summary of the results of the build (if available):

== Tests result: FAILURE then FAILURE ==

429 tests OK.

1 test failed:
test.test_asyncio.test_subprocess

17 tests skipped:
test.test_asyncio.test_windows_events
test.test_asyncio.test_windows_utils test_asdl_parser test_clinic
test_devpoll test_gdb test_generated_cases test_ioctl test_kqueue
test_launcher test_perf_profiler test_startfile test_winconsoleio
test_winreg test_winsound test_wmi test_zipfile64

1 re-run test:
test.test_asyncio.test_subprocess

Total duration: 8 min 58 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.13/test/test_asyncio/test_subprocess.py", line 788, in test_subprocess_consistent_callbacks
    self.loop.run_until_complete(main())
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.13/asyncio/base_events.py", line 664, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.13/test/test_asyncio/test_subprocess.py", line 780, in main
    self.assertEqual(events, [
AssertionError: Lists differ: ['process_exited', ('pipe_data_received', 1, b'stdout')] != [('pipe_data_received', 1, b'stdout'), ('p[95 chars]ted']