[Bug target/113960] std::map with std::vector as input overwrites itself with c++20, on s390x platform

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113960

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But it's guarded by:

        if constexpr (__is_byte_iter<_InputIter1>)
          if constexpr (__is_byte_iter<_InputIter2>)

This condition is only supposed to be true when sizeof(*__first1) == 1 and
sizeof(*__first2) == 1

We can only use memcmp if we're comparing single bytes as unsigned values (and
if the iterators are pointers to contiguous memory, not e.g. segmented
iterators like std::deque's, or not even random access iterators, like
std::list's).

For std::vector<unsigned short> we should not use this code at all.