[Bug libstdc++/110512] C++20 random access iterators run sequentially with PSTL

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110512

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Actually, if we don't care about upstream any more, we can improve the
pre-C++20 version too:

template <typename... _IteratorTypes>
using __are_random_access_iterators
    = std::__or_<std::is_base_of<std::random_access_iterator_tag,
                                 std::__iter_category_t<_IteratorTypes>>...>;