[Bug c++/81944] New: constexpr std::distance

["alexbaroni68 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81944

            Bug ID: 81944
           Summary: constexpr std::distance
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alexbaroni68 at gmail dot com
  Target Milestone: ---

When using the constexpr overload of std::distance it complains that a not
constexpr function is called, follows code to reproduce and error message:

#include <iostream>
#include <algorithm>
#include <iterator>
#include <stdexcept>
#include <array>
#include <string>
#include <cassert>

template<typename It>
constexpr auto max (It first, It last)
{
  auto count = std::distance(first, last);
  if (count == 0)
    throw std::domain_error{"max is undefined on empty range"};
  if (count == 1) return *first;
  auto mid = first + (last - first) / 2;
  return std::max(max(first, mid), max(mid, last));
}

int main()
{
  constexpr std::array<int, 4> vs{1,6,8,3};
  constexpr auto max_of_vs = max(std::cbegin(vs), std::cend(vs));
  assert(max_of_vs == 8);
  std::cout << max_of_vs << std::endl;
  std::array<int, 5> xs{1,9,8,3,6};
  auto max_of_xs = max(std::begin(xs), std::end(xs));
  assert(max_of_xs == 9);
  std::cout << max_of_xs << std::endl;
  //std::array<int, 0> zs{};
  //auto max_of_zs = max(std::begin(zs), std::end(zs)); // throws at runtime
  //assert(max_of_zs == 0);
}


main.cpp: In function 'int main()':

main.cpp:23:33:   in constexpr expansion of 'max<const
int*>(std::cbegin<std::array<int, 4> >(vs), std::cend<std::array<int, 4>
>(vs))'

main.cpp:12:29: error: 'constexpr typename
std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator,
_InputIterator) [with _InputIterator = const int*; typename
std::iterator_traits<_Iterator>::difference_type = long int]' called in a
constant expression

   auto count = std::distance(first, last);

                ~~~~~~~~~~~~~^~~~~~~~~~~~~

In file included from /usr/local/include/c++/7.2.0/bits/stl_algobase.h:66:0,

                 from /usr/local/include/c++/7.2.0/bits/char_traits.h:39,

                 from /usr/local/include/c++/7.2.0/ios:40,

                 from /usr/local/include/c++/7.2.0/ostream:38,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/stl_iterator_base_funcs.h:138:5: note:
'constexpr typename std::iterator_traits<_Iterator>::difference_type
std::distance(_InputIterator, _InputIterator) [with _InputIterator = const
int*; typename std::iterator_traits<_Iterator>::difference_type = long int]' is
not usable as a constexpr function because:

     distance(_InputIterator __first, _InputIterator __last)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/bits/stl_iterator_base_funcs.h:142:33: error: call
to non-constexpr function 'typename
std::iterator_traits<_Iterator>::iterator_category
std::__iterator_category(const _Iter&) [with _Iter = const int*; typename
std::iterator_traits<_Iterator>::iterator_category =
std::random_access_iterator_tag]'

         std::__iterator_category(__first));

         ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~