cmath.acosh(complex('±0+nanj')) should return complex('nan±pi/2j') (imaginary part sign is unspecified)

[skirpichev]

Bug report

Bug description:

As per C11 DR#471 (adjusted resolution accepted for C17). See glibc patch: https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=5244527da1bfa751492e146d3e6ae128c59bfcc4

Currently, imaginary part is nan.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-121889: cmath.acosh(0+nanj) returns nan+pi/2j #121892

[mdickinson]

Fixed in #121892 (with thanks to @skirpichev). As commented on the PR, I don't think this should be backported.