Method parse

extlibs/fmt/include/fmt/format.h:3078–3083

 public:
  template <typename ParseContext>
  auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
    format_str_ = ctx.begin();
    // Checks are deferred to formatting time when the argument type is known.
    internal::dynamic_specs_handler<ParseContext> handler(specs_, ctx);
    return parse_format_specs(ctx.begin(), ctx.end(), handler);
  }

  template <typename T, typename FormatContext>
  auto format(const T& val, FormatContext& ctx) -> decltype(ctx.out()) {