Hi, I updated my PEP 7 PR to use C99 in the public C API and "a subset of" C11 in Python internals: "Python 3.11 and newer versions use C99 in the public C API and use a subset of C11 in Python internals. The public C API should be compatible with C++. The C11 subset are features supported by GCC 8.5, clang 8.0, and MSVC of Visual Studio 2017." https://github.com/python/peps/pull/2309/files GCC 8.5 is the version chosen by RHEL 8. It should provide C11 features that we care about. I pickled clang 8.0 because it's had been released in 2019 and so should be available on most operating systems. FreeBSD uses clang by default. FreeBSD 13 uses clang 11. And it seems like we still care about support Visual Studio 2017, even if Visual Studio 2019 and 2022 are available. I chose to not require supporting AIX XLC. Inada-san wrote: "xlclang fully supports C89/C99/C11. xlc fully supports C89/C99, and partially supports C11." I guess that in practice, we can test a PR on buildbots when trying "new shiny" C11 feature. Moreover, if a C11 feature is missing, it's usually not too complicated to use a workaround for C99 and older. Victor