[Bug libstdc++/107367] New: All standard library algorithms should detect whether they are contiguous iterators after C++20

["unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107367

            Bug ID: 107367
           Summary: All standard library algorithms should detect whether
                    they are contiguous iterators after C++20
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: unlvsur at live dot com
  Target Milestone: ---

https://github.com/gcc-mirror/gcc/blob/00716b776200c2de6813ce706d2757eec4cb2735/libstdc%2B%2B-v3/include/bits/stl_algo.h#L3229

Take is_sorted for example:

  template<typename _ForwardIterator>
    _GLIBCXX20_CONSTEXPR
    inline bool
    is_sorted(_ForwardIterator __first, _ForwardIterator __last)
    { return std::is_sorted_until(__first, __last) == __last; }

It should be optimized to

  template<typename _ForwardIterator>
    _GLIBCXX20_CONSTEXPR
    inline bool
    is_sorted(_ForwardIterator __first, _ForwardIterator __last)
    { 
       if
constexpr(contiguous_iterator<_ForwardIterator>&&!is_pointer_v<_ForwardIterator>)
{
       return is_sorted(to_address(__first),to_address(__last));
}
else
{
 return std::is_sorted_until(__first, __last) == __last; 
}
 }