Bug report

Bug description:

Include/pyport.h currently only guards use of __attribute__((fallthrough)) with _Py__has_attribute(fallthrough). Clang versions prior to 10 do not support this syntax, but do claim to support the fallthrough attribute (because they support the C++11 style [[fallthrough]]).

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-128902: Fix check for fallthrough attribute support #128903