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From: "reconnoo at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/104801] New: std::ranges::view_interface::empty is
 overconstrained
Date: Sat, 05 Mar 2022 21:16:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104801

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

{
  auto [begin, end] =3D std::subrange(v);
  auto is_empty =3D begin =3D=3D end;  // works
}=