https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99979
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- The _Unlock type was introduced for PR libstdc++/50862 (and then modified slightly by PR libstdc++/53830).
redi at gcc dot gnu.org via Gcc-bugs Thu, 08 Apr 2021 16:44:21 -0700
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99979
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- The _Unlock type was introduced for PR libstdc++/50862 (and then modified slightly by PR libstdc++/53830).