GeezerVison

I wanted to use PrepareForClangClBuild here like @zooba suggested, which would have saved the extra $(PlatformToolset) == 'ClangCL', but unfortunately this did not work.

It can be a little less reliable, but doing the comparison with $([System.Version]::Parse($(LLVMToolsVersion))) < $([System.Version]::Parse('19.0')) will at least ensure we're comparing version numbers and not garbage.

We probably also need to put the setting (and warning) in pyproject.props, to make sure that any use in any extension modules is also handled. In general, everything that applies to pythoncore.vcxproj also applies to extension modules, which is why most of the settings are in the shared file.

IIUC, you mean, if we're compiling one file with /arch:AVX, we can do for all - pythonxxx.dll will anyway have this run time dependency then?

So let's move the setting to pyproject.props - and if #129907 is merged, this will probably move to pyproject-clangcl.props?

If I am not mistaken, by moving the warning, we'd see it when building any extension module during the build process, i.e. many times. Doesn't hurt IMHO, is quite an important warning - isn't it?

The reasons we need the flag could potentially apply to an extension module - e.g. if someone moves the blake module to its own file (probably where it ought to be anyway). So we ought to turn it on for everything.

Warning for everything isn't quite as important - one per build is probably fine - but it's easier to define it all in one place, and getting the warning multiple times isn't bad, so may as well keep it simple.

I'm pretty sure we'll get the other one merged soon (I haven't looked at it yet this week), so might get to move it all into pyproject-clangcl.props before merging this one anyway.

@msprotz is working on hiding the "problematic" #include "libintvector.h" (#130483 (comment)), so most probably we won't need it just as a temporary workaround.

In the meantime, the few people using clang-cl should use versions >= 19.0.0. They'd likely do anyway, to use the tailcalling interpreter (#130040).