Function parse_arg_id

extlibs/fmt/include/fmt/format.h:2336–2366

template <typename Char, typename IDHandler>
FMT_CONSTEXPR const Char* parse_arg_id(const Char* begin, const Char* end,
                                       IDHandler&& handler) {
  FMT_ASSERT(begin != end, "");
  Char c = *begin;
  if (c == '}' || c == ':') {
    handler();
    return begin;
  }
  if (c >= '0' && c <= '9') {
    int index = 0;
    if (c != '0')
      index = parse_nonnegative_int(begin, end, handler);
    else
      ++begin;
    if (begin == end || (*begin != '}' && *begin != ':'))
      handler.on_error("invalid format string");
    else
      handler(index);
    return begin;
  }
  if (!is_name_start(c)) {
    handler.on_error("invalid format string");
    return begin;
  }
  auto it = begin;
  do {
    ++it;
  } while (it != end && (is_name_start(c = *it) || ('0' <= c && c <= '9')));
  handler(basic_string_view<Char>(begin, to_unsigned(it - begin)));
  return it;
}

// Adapts SpecHandler to IDHandler API for dynamic width.
template <typename SpecHandler, typename Char> struct width_adapter {