[Bug libstdc++/104801] New: std::ranges::view_interface::empty is overconstrained

[reconnoo at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104801

            Bug ID: 104801
           Summary: std::ranges::view_interface::empty is overconstrained
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reconnoo at gmail dot com
  Target Milestone: ---

I can't see why empty() needs a forward_iterator. It looks overconstrained and
input_iterator should suffice because it is equivalent to the begin/end
iterator comparison, yet, somehow this does not compile:

auto is = std::istringstream{"1 2 3"};
auto v = ranges::istream_view<int>(is);
{
  auto is_empty = v.empty();  // error
}

{
  auto [begin, end] = std::subrange(v);
  auto is_empty = begin == end;  // works
}