The behaviour of the default constructor `std::variant<T0, ...>` is as follows:

* (a) If `T0` is default constructible, then the `variant` will hold
  a value-initialized `T0` and the index is 0.
* (b) If `T0` is not default constructible, then the default constructor
  does npt participate in overload resolution.

In contrast to this until now the following is implemented

* (c) The default constructor of `Std::variant` does alway exist
  and will initialize it with a special invalid state (which
  is BTW not copyable).

Having (c) instead of (a) especially prevented copying default
constructed `Std::variant<Std::monostate, ...>` objects.

This patch at least ensures (a) but keeps (c) if `T0` is not
default constructible. The reason for the latter is, that (b)
is much harder to fix and unlikely to lead to problems.