This is an implemention of optional from the standard library. If it detects that a proper optional exists, it will attempt to use it. This is mostly an implementation detail, used in the :ref:`sol::stack::check_get<stack-check-get>` and :ref:`sol::stack::get\<optional\<T>><stack-get>` and sol::optional<T> maybe_value = table["arf"]; implementations for additional safety reasons.
See this example here for a demonstration on how to use it and other features!