[Bug libstdc++/114645] std::chrono::current_zone ignores $TZ on Linux

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114645

--- Comment #23 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Re https://github.com/cplusplus/draft/issues/6922

It can't possibly mean that the returned time zone *needs* to be the same as
the C library, because that's impossible in general, because the C library
isn't required to use IANA zones at all. See my TZ=garbage23nonsense example.
There is no way to match that with std::chrono::time_zone without pointer
lifetime issues and inconsistencies like returning a time_zone that isn't in
the tzdb.

And if the C++ standard intended to require that TZ is respected on POSIX
systems, then it would say so. The absence of any requirement means it's not
required. TZ is not mentioned in the standard at all. The standard probably
isn't going to say it *must not* be used, because it's not in the business of
listing (or forbidding) possible vendor extensions. There might be systems
where a TZ environment var is the only way the time zone can be set, and it
always is an IANA zone. But I don't believe any of the targets libstdc++
supports fit into that category.