[Bug libstdc++/115799] New: ranges::find's optimized branching for memchr is not quite right

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115799

            Bug ID: 115799
           Summary: ranges::find's optimized branching for memchr is not
                    quite right
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

ranges_util.h#L505:

        if constexpr (is_same_v<_Proj, identity>)
          if constexpr(__can_use_memchr_for_find<iter_value_t<_Iter>, _Tp>)
            if constexpr (sized_sentinel_for<_Sent, _Iter>)
              if constexpr (contiguous_iterator<_Iter>)
                if (!is_constant_evaluated())
                  {
                    if (static_cast<iter_value_t<_Iter>>(__value) != __value)
                      return __last;


We should return __first + __n as __last may not be convertible to __first, for
example:


  std::string s;
  auto r = std::ranges::find(std::counted_iterator{s.begin(), 3},
                             std::default_sentinel, 
                             ' ');