Method on_format_specs

extlibs/fmt/include/fmt/format.h:2667–2670

  FMT_CONSTEXPR void on_replacement_field(const Char*) {}

  FMT_CONSTEXPR const Char* on_format_specs(const Char* begin, const Char*) {
    advance_to(context_, begin);
    return arg_id_ < num_args ? parse_funcs_[arg_id_](context_) : begin;
  }

  FMT_CONSTEXPR void on_error(const char* message) {
    context_.on_error(message);