Return type std::optional<std::variant<...>>

I would consider this to be a useful use of std::monostate. Specifically, variant<std::monostate, int, double, std::string, std::chrono::time_point>. monostate is useful for cases where a variant may not contain a value.

The nice thing about using an actual type rather than optional<variant> is that visitation works normally on it. You can write a functor that can take a monostate parameter, thus allowing you to use visit for even "empty" variants.

Just want to add that before C++17 and the standardization of variant and monostate, there is already boost::blank to solve the exact same issue for boost::variant.

By convention, if boost::blank is used, it should always be the first template argument, so that a default-constructed variant is empty and checking for emptyness is done with .which() == 0.