Optional<T> equivalent in D?

[bearophile]

Justin Whear:

> No, Nullable adds a potential null state to value types, e.g. 
> it allows you to null an int.

In std.typecons there is another version of Nullable, that uses a 
state as "null". So you can use it as nullable class reference. 
Is that good enough for the OP?

Bye,
bearophile