This is a consequence of the experiments with generic lambdas.
Up to now, lib::meta::_Fun<F> failed with a compilation error
when passing the decltype of such a generic lambda.
The new behaviour is to pick the empty specialisation (std::false_type) in such cases,
allowing to guard explicit specialisations when no suitable functor type
is passed
...since there is not any test coverage for this trait, which
turned out to be quite deeply rooted in the system by now and
handles several rather subtle special cases
