[Bug libstdc++/77744] Data race on std::regex_iterator using openmp

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77744

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The std::thread example is similar, but not the same. The OpenMP version shares
f, so is equivalent to creating the std::thread objects with std::ref(f).

More importantly, the OpenMP version does not wait for the loop to finish (or
at least, I see "#pragma omp for nowait" in the GIMPLE dump), so there is no
happens-before relation between the loop body and the reTest destructor.

I see the same races using:

    for(unsigned i = 0; i < 10; ++i)
        std::thread{std::ref(f)}.detach();