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
}