https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100117
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- <iostream> doesn't include <csignal> AFAIK. <bits/stdc++.h> is a non-standard header that just includes all standard C++ headers (except for <execution> it seems). And it is <csignal> that now newly (in glibc 2.34) indirectly includes <unistd.h> when it didn't before.
