Function underlying

include/fmt/ranges.h:417–419

  FMT_CONSTEXPR range_formatter() {}

  FMT_CONSTEXPR auto underlying() -> detail::range_formatter_type<Char, T>& {
    return underlying_;
  }

  FMT_CONSTEXPR void set_separator(basic_string_view<Char> sep) {
    separator_ = sep;