message: rvalues should also be tested for writability, refs ericniebler/stl2#387
diff: @@ -178,10 +178,12 @@ namespace ranges
  using reference_t = Readable::reference_t<Out>;
  template<typename Out, typename T>
  - auto requires_(Out& o, T &&t) -> decltype(
  + auto requires_(Out&& o, T &&t) -> decltype(
  concepts::valid_expr(
  ((void)(*o = (T &&) t), 42),
  - ((void)(const_cast<reference_t<Out> const &&>(*o) = (T &&)t), 42)
  + ((void)(*((Out &&) o) = (T &&) t), 42),
  + ((void)(const_cast<reference_t<Out> const &&>(*o) = (T &&)t), 42),
  + ((void)(const_cast<reference_t<Out> const &&>(*((Out &&) o)) = (T &&)t), 42)
  ));
  };
  