[Bug c++/104461] cody requires -fmodule-mapper hostname to have an IPv6 address

[xry111 at mengyan1223 dot wang]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104461

Xi Ruoyao <xry111 at mengyan1223 dot wang> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jwakely.gcc at gmail dot com,
                   |                            |xry111 at mengyan1223 dot wang

--- Comment #1 from Xi Ruoyao <xry111 at mengyan1223 dot wang> ---
It's because the paper (https://wg21.link/p1184) explicitly says "an ipv6
domain socket & port", I think.  It's not clear if AI_V4MAPPED addresses are
allowed.

The problem is there is no "elegent" way for a distro maintainer to ensure an
IPv6 address for localhost is available on Linux/GNU.  If /etc/nsswitch.conf
contains

hosts: files dns

(this is the default of Glibc and used by many distros), and /etc/hosts
contains

127.0.0.1 localhost
::1       localhost

By default Glibc will ignore "::1" for localhost, unless "multi on" is
specified in /etc/host.conf.  But Glibc doc explicitly says it may cause a
performance issue (see man host.conf).  On systems using systemd, the distro
can specify nss_myhostname in /etc/nsswitch.conf, but it does not exist on
non-systemd distros.