The PEP 7 documents, that we must use C11 (without optional features) since CPython 3.11. erf, erfc, lgamma and tgamma (math.gamma) are part of the C99 (and mandatory for C11 too).

Probably, it's not a bad idea to get rid of custom implementations of such functions or document why it's present (e.g. for some exotic platform, broken libc implementation, speed optimization, etc). Keep in mind, that this code isn't tested by CI if we optionally include using standard implementations (as for erf with #ifdef HAVE_ERF).

Linked PRs

• gh-101678: refactor the math module to use special functions from c11 #101679
• gh-101678: Merge math_1_to_whatever() and math_1() #101730