Command Line Parser with C++26 reflections

[the-moisrex]

From P2996R4 and its second implementation:

template<typename Opts>
auto parse_options(std::span<std::string_view const> args) -> Opts {
  Opts opts;
  template for (constexpr auto dm : nonstatic_data_members_of(^Opts)) {
    auto it = std::ranges::find_if(args,
      [](std::string_view arg){
        return arg.starts_with("--") && arg.substr(2) == name_of(dm);
      });

    if (it == args.end()) {
      // no option provided, use default
      continue;
    } else if (it + 1 == args.end()) {
      std::print(stderr, "Option {} is missing a value\n", *it);
      std::exit(EXIT_FAILURE);
    }

    using T = typename[:type_of(dm):];
    auto iss = std::ispanstream(it[1]);
    if (iss >> opts.[:dm:]; !iss) {
      std::print(stderr, "Failed to parse option {} into a {}\n", *it, display_name_of(^T));
      std::exit(EXIT_FAILURE);
    }
  }
  return opts;
}

struct MyOpts {
  std::string file_name = "input.txt";  // Option "--file_name <string>"
  int    count = 1;                     // Option "--count <int>"
};

int main(int argc, char *argv[]) {
  MyOpts opts = parse_options<MyOpts>(std::vector<std::string_view>(argv+1, argv+argc));
  // ...
}

This idea seems really cool for the users to have.