[Bug libstdc++/91910] Debug mode: there is a racing condition between destructors of iterator and the associated container.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91910

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Specifically, [res.on.data.races] p7 in the standard says we need to protect
the container's internal state from data races.