[Bug libstdc++/91486] future::wait_for and shared_timed_mutex::wait_for do not work properly with float duration

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91486

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ah yes, of course. Thanks!