Hi, I made a change to require C99 <math.h> "NAN" constant and I'm was asked to update the PEP 7 to clarify the C subset is needed to build Python 3.11. Python 3.6 requires a subset of the C99 standard to build defined by the PEP 7: https://www.python.org/dev/peps/pep-0007/ I modified Python 3.11 to require more C99 features of the <math.h> header: * bpo-45440: copysign(), hypot(), isfinite(), isinf(), isnan(), round() * bpo-46640: NAN After my NAN change (bpo-46640), Petr Viktorin asked me to update the PEP 7. I proposed a change to simply say that "Python 3.11 and newer versions use C99": https://github.com/python/peps/pull/2309 I would prefer to not have to give an exhaustive list of C99 features used by CPython, since it's unclear to me what belongs to C99 or to ISO C89. As I wrote before, Python already uses C99 features since Python 3.6. On my PEP PR, Guido van Rossum asked me to escalate the discussion to python-dev, so here I am :-) In "C99", the number "99" refers to the year 1999, the standard is now 23 years old: https://en.wikipedia.org/wiki/C99 In 2022, C99 is now well supported by C compilers supported by Python: GCC, clang, MSVC. I don't know if AIX XLC supports C99. AIX provides a "c99" compiler compatible with C99. It also seems like GCC is usable on AIX. I don't know if ICC supports C99. Python doesn't officially the ICC compiler, the ICC buildbots are gone a few years ago. But sometimes I make some changes to enhance the ICC support, when the change is small enough. Note: Python also uses C11 <stdatomic.h>, but it's not required: there are fallbacks for compilers which don't support it. Victor -- Night gathers, and now my watch begins. It shall not end until my death.