Std :: iterator_traits discrepancy between libstdc ++ and libc ++

Question

Given:

struct Iter {
    using value_type = int;
    using difference_type = int;
    using reference = int;
    using pointer = int;
    using iterator_category = int;
};

The following works fine with libstC ++, but does not compile in libC ++ 5.0.0:

#include <iterator>
#include <type_traits>
static_assert(
    std::is_same<
        std::iterator_traits<Iter>::iterator_category,
        Iter::iterator_category
    >::value, "");

With an error:

error: no member named ' iterator_category' in ' std::__1::iterator_traits<Iter>'         std::is_same<std::iterator_traits<Iter>::iterator_category, Iter::iterator_category>::value, "");

A static statement succeeds if it Iter::iterator_categoryis one of the standard input categories, for example. std::input_iterator_tag.

IMHO this should not fail, because the C ++ project is indicated in [iterator.traits] # ​​2 :

If Iterator has a valid ( [temp.deduct] ) types of members difference_­type, value_­type, pointer, reference, and iterator_­category, iterator_­traits<Iterator>should have the following public items:

using difference_type   = typename Iterator::difference_type;
using value_type        = typename Iterator::value_type;
using pointer           = typename Iterator::pointer;
using reference         = typename Iterator::reference;
using iterator_category = typename Iterator::iterator_category;

iterator_­traits<Iterator> .

- , ?

Answer 1

[std.iterator.tags]:

, , , . , , . : input_­iterator_­tag, output_­iterator_­tag, forward_­iterator_­tag, bidirectional_­iterator_­tag random_­acces_­iterator_­tag. Iterator, iterator_­traits<Iterator>​::​iterator_­category , .

namespace std {
  struct input_iterator_tag { };
  struct output_iterator_tag { };
  struct forward_iterator_tag: public input_iterator_tag { };
  struct bidirectional_iterator_tag: public forward_iterator_tag { };
  struct random_access_iterator_tag: public bidirectional_iterator_tag { };
}

int , iterator_traits<Iter>::iterator_category int. , iterator_traits - , , , .

[], . , , lib++ - iterator_traits<Iter>, , .