@erikjanss In my interpretation, this is already implied as the PEP mentions ANSI/ISO standard C (though since the list of C99 features was added, it became a little less clear).
Do you have a specific example of a MSVC extension that should be called out? Is there non-standard code in CPython now that you'd like to get rid of?

@encukou Up to now I only found one (I have not yet been able to go through all files though),
since the PEP mentions GCC explicitly, it might be read as 'MSVC extension is fine in windows specific code'

Modules/socketmodule.c line 562 :

# define SUPPRESS_DEPRECATED_CALL __pragma(warning(suppress: 4996))

uses __pragma, which appears to be a MSVC extension :

https://msdn.microsoft.com/en-us/library/d9x1s805.aspx

Can you file a CPython pull request to properly conditionalize that?
I don't know much about coding for Windows, but to me this looks like an oversight rather than deliberately equating MS_WINDOWS with MSVC.

I think it will be useful to first have a PR discussion on this particular case, and then to clarify the PEP based on the PR discussion.

I made a PR for this :

python/cpython#9067

The associated issue is 34581

https://bugs.python.org/issue34581