[Bug libstdc++/95983] New: `std::counted_iterator<std::iterator_t<std::ranges::basic_istream_view<...>>>` fails to satisfy `std::input_or_output_iterator`

["ensadc at mailnesia dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95983

            Bug ID: 95983
           Summary: `std::counted_iterator<std::iterator_t<std::ranges::ba
                    sic_istream_view<...>>>` fails to satisfy
                    `std::input_or_output_iterator`
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
  Target Milestone: ---

https://wandbox.org/permlink/rUZlUGG4UFeWicbM
====
#include <ranges>
#include <iterator>
#include <iostream>
using R =
  std::ranges::basic_istream_view<int, char, std::char_traits<char>>;
using It = std::ranges::iterator_t<R>;
static_assert(std::input_or_output_iterator<std::counted_iterator<It>>);

====
prog.cc:5:20: error: static assertion failed
    5 | static_assert(std::input_or_output_iterator<std::counted_iterator<T>>);
      |               ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:5:20: note: constraints not satisfied
In file included from
/opt/wandbox/gcc-head/include/c++/11.0.0/bits/stl_iterator_base_types.h:71,
                 from /opt/wandbox/gcc-head/include/c++/11.0.0/iterator:61,
                 from /opt/wandbox/gcc-head/include/c++/11.0.0/ranges:44,
                 from prog.cc:1:
/opt/wandbox/gcc-head/include/c++/11.0.0/bits/iterator_concepts.h:536:13:  
required for the satisfaction of 'weakly_incrementable<_Iter>' [with _Iter =
std::counted_iterator<std::ranges::basic_istream_view<int, char,
std::char_traits<char> >::_Iterator<int, char, std::char_traits<char> > >]
/opt/wandbox/gcc-head/include/c++/11.0.0/bits/iterator_concepts.h:538:10:   in
requirements with '_Iter __i' [with _Tp =
std::counted_iterator<std::ranges::basic_istream_view<int, char,
std::char_traits<char> >::_Iterator<int, char, std::char_traits<char> > >;
_Iter = std::counted_iterator<std::ranges::basic_istream_view<int, char,
std::char_traits<char> >::_Iterator<int, char, std::char_traits<char> > >]
/opt/wandbox/gcc-head/include/c++/11.0.0/bits/iterator_concepts.h:540:11: note:
the required type 'std::iter_difference_t<_Iter>' is invalid
  540 |  typename iter_difference_t<_Iter>;
      |  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
/opt/wandbox/gcc-head/include/c++/11.0.0/bits/iterator_concepts.h:541:21: note:
nested requirement '__is_signed_integer_like<typename
std::__detail::__iter_traits_impl<typename std::remove_cv<typename
std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename
std::remove_cv<typename std::remove_reference<_Tp>::type>::type>
>::type::difference_type>' is not satisfied
  541 |  requires __detail::__is_signed_integer_like<iter_difference_t<_Iter>>;
      |           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more
detail

====
It seems that in this case, `iter_difference_t` is treated as an alias of
`std::iterator_traits<std::counted_iterator<It>>::difference_type` (which
doesn't exist), because libstdc++ defines a specialization of
`std::iterator_traits<std::counted_iterator<It>>`.

(The error message is worse than it should be, due to bug 94862 and bug 95303.)