action that returns nothing

[zhihaoy]

Currently this code isn't allowed in argparse:

program.add_argument(...)
    .action([](std::string const& filename)
    {
        load_cfg(filename);
    });

Because argparse internal expects action to return some value, but this one returns void.
My workaround for that is to define using unit = std::tuple<>; and returns unit(), but this isn't nice to C++'s taste.
But it's possible to bake this workaround into argparse (even though there is no any<void>). Let's say we define a type similar to the unit above, call is none_internal. In action, detects the the input's return type:

  template<class F>
  Argument &action(F&& f) {
    if constexpr (std::is_void_v<std::invoke_result_t<F, std::string>>)
      mAction = [f](std::string const& arg) { f(arg); return none_internal(); };
    else
      mAction = std::move(aAction);
    return *this;
  }