message: add missing constraint to view::sliding
diff: @@ -359,7 +359,7 @@ namespace ranges
  template<typename Rng>
  auto operator()(Rng &&rng, range_difference_t<Rng> n) const ->
  CPP_ret(sliding_view<all_t<Rng>>)(
  - requires ForwardRange<Rng>)
  + requires ViewableRange<Rng> && ForwardRange<Rng>)
  {
  return {all(static_cast<Rng &&>(rng)), n};
  }
  