Method format_arg get_arg

extlibs/fmt/include/fmt/format.h:2195–2199

template <typename Context>
FMT_CONSTEXPR typename Context::format_arg get_arg(Context& ctx, int id) {
  auto arg = ctx.arg(id);
  if (!arg) ctx.on_error("argument index out of range");
  return arg;
}

// The standard format specifier handler with checking.
template <typename ParseContext, typename Context>