Silence experimental coroutine deprecation warnings

[hugovk]

The Windows release build fails for today's 3.15 beta 2:

C:\...\MSVC\14.51.36231\include\experimental\coroutine(37,1): error C2338:
static assertion failed: 'error STL1011: The /await compiler option,
<experimental/coroutine>, <experimental/generator>, and <experimental/resumable>
are deprecated by Microsoft and will be REMOVED SOON. ... You can define
_SILENCE_EXPERIMENTAL_COROUTINE_DEPRECATION_WARNINGS to suppress this error for now.'

https://dev.azure.com/Python/cpython/_build/results?buildId=170819&view=logs&j=0108ec1a-385a-59f8-f3cd-2c3a968973ca&t=e19eaefe-6260-5118-3fc8-77764c035640

Here's Claude's summary, and it affects more than free-threaded debug AMD64. This PR does fix number 1.

---

Build: Azure Pipelines build 170819 (Python 3.15.0b2, free-threaded debug AMD64), 2026-06-02.

Symptom

python_uwp.vcxproj fails at the very end of the build:

C:\...\MSVC\14.51.36231\include\experimental\coroutine(37,1): error C2338:
static assertion failed: 'error STL1011: The /await compiler option,
<experimental/coroutine>, <experimental/generator>, and <experimental/resumable>
are deprecated by Microsoft and will be REMOVED SOON. ... You can define
_SILENCE_EXPERIMENTAL_COROUTINE_DEPRECATION_WARNINGS to suppress this error for now.'

Root cause

Toolchain bump on the agent image — not a code regression.

Build	Date	Toolchain	Result
Previous	2026-05-10	VS 2022 / MSVC 14.44.35207	✅ passes (deprecation = warning)
Failing	2026-06-02	VS 18 / MSVC 14.51.36231	❌ fails (deprecation = static_assert)

PC/python_uwp.cpp transitively pulls in <experimental/coroutine> via its <winrt/...> includes under
/std:c++17. The file already silences the clang-equivalent warning but has no MSVC silencer.

Why the toolchain changed

windows-release/azure-pipelines.yml:174 (added in release-tools PR python/release-tools#346, Support optionally
building AMD64 with tailcalling) routes tail-calling builds to the windows-2025-vs2026 agent image,
which ships VS 18:

vmImage: ${{ iif(eq(parameters.DoTailCalling, 'true'),
                  'windows-2025-vs2026',
                  parameters.vmImage) }}

Non-tailcall builds still go to the default windows-2025 (VS 2022) image and are unaffected.

Fix options (upstream CPython, not release-tools)

1. Quick — add _SILENCE_EXPERIMENTAL_COROUTINE_DEPRECATION_WARNINGS to PC/python_uwp.cpp
   (matching the existing clang silencer) or to the preprocessor defines in PCbuild/python_uwp.vcxproj and
   PCbuild/pythonw_uwp.vcxproj.
2. Proper — bump those two vcxprojs from /std:c++17 to /std:c++20; modern C++/WinRT uses the
   standard <coroutine> header in C++20 mode. PCbuild/_wmi.vcxproj is already on C++20.

Workaround to ship now

Queue the release build without DoTailCalling=true so it lands on the VS 2022 agent.

[zware]

Looks fine to me, at least as a quick fix for a beta release :)

We recently got around this in CI by just disabling the UWP build entirely. I'm not up to date on whether we should just rip out the UWP entirely, but that might be an option for a permanent fix.

[zooba]

We can't rip it out entirely yet, since 3.13 is still going to the Store, but disabling that part of the build for our release process is the way I'd do it. Suppressing the warning is fine too, if it works.

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-150794 is a backport of this pull request to the 3.15 branch.

[chris-eibl]

This nicely fits the silence I had to already do a while ago for clang :)