Class accessor

extlibs/fmt/include/fmt/core.h:746–749

inline Container& get_container(std::back_insert_iterator<Container> it) {
  using bi_iterator = std::back_insert_iterator<Container>;
  struct accessor : bi_iterator {
    accessor(bi_iterator iter) : bi_iterator(iter) {}
    using bi_iterator::container;
  };
  return *accessor(it).container;
}