László, sorry, hadn't seen activity on fetchmail in a long time.
My apologies to you for my false statement about your status, and thanks for the prompt upload.
I'd found the bug only yesterday (after checking fetchmail's SMTP AUTH code when I saw the mutt 2.2.15 release announcement that added a workaround for Microsoft-ish not-quite-RFC-complient SASL).
Regarding the older releases (6.4.X), I have a patch linked in my original bug report, or you can review the 2025-10-03 patches from <https://gitlab.com/fetchmail/fetchmail/-/commits/legacy_64?ref_type=heads> and see what you'll need. I suppose the prototype/C23 changes are irrelevant for now.
Canonical workaround for C23 incompatibilites (that might disable DNS-related options) if someone compiles with a compiler (GCC15) defaulting to C23 is avoiding just that and using -std=gnu17 or -std=c17 instead.
Cheers, Matthias
