message: view::concat(r) should be equivalent to view::all(r); refs
diff: @@ -393,6 +393,13 @@ namespace ranges
  {
  return concat_view<all_t<Rngs>...>{all(static_cast<Rngs &&>(rngs))...};
  }
  + template<typename Rng>
  + auto operator()(Rng && rng) const //
  + -> CPP_ret(all_t<Rng>)( //
  + requires viewable_range<Rng> && input_range<Rng>)
  + {
  + return all(static_cast<Rng &&>(rng));
  + }
  };
  /// \relates concat_fn
  