I'm a bit sceptical.

Portability: While GetAddrInfoW is supported on Windows, there is also an extension (via the AI_IDN flag and more flags) for GNU/Linux (but there is no POSIX extension for this - i.e. no way to do this (with libc only) on FreeBSD/OpenBSD/MacOSX). What C applications usually do is to link against libidn (which I do not suggest the OCaml runtime to do).

This means this PR will introduce divergent behaviour between platforms. I'd instead document that IDNs are not supported in the stdlib. There exists an (unreleased) library which does the punycode conversion in pure OCaml. Doing this depends on a unicode library, thus it is not suitable for the stdlib IIUC.

OTOH, if divergence between platforms is not an issue, surely this is ok.

IDN stuff needs to be considered very, very carefully when it is integrated because of the security issues that have been mentioned.

I note that OCaml's own unicode support is not yet quite complete, which makes it harder to figure out a good solution that works for all of OCaml's platforms.

@hannesm I was not aware of the AI_IDN flag for GNU/Linux and the divergence this PR would introduce, thanks for bringing it to my attention.

Technically speaking the divergence is already there today, because it is possible to use Latin-1 (or local code page encoded) hostnames under Windows but only ASCII under Linux (if I understand correctly).

I do not have a strong argument for merging apart from coherence with the rest of the win32unix library which has been shifted from code page encoding to UTF-8. I do not understand all the security implications of merging this at the moment, so I will happily follow the lead of anyone who does.

There does not seem to be a consensus for merging this PR, so closing for now. Can always be reopened if needed.