[Bug libstdc++/108211] std::chrono::current_zone() fails if zone only has one component

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108211

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The obvious solution is to try locate_zone(dir/filename) and if that fails try
locate_zone(filename).