Nullable with reference types

[via Digitalmars-d-learn]

On Tuesday, 30 June 2015 at 18:29:31 UTC, Steven Schveighoffer 
wrote:
> I know this is just back-of-envelope, but what's wrong with:
>
> alias Nullable(T) if(is(T == class)) = T;
>
> bool isNull(T)(T t) if(is(T == class)) { return t is null;}

That's what I intended. (Same for pointers and slices, BTW.)

I does however have a slightly different behaviour: In the 
current implementation, there can be instances for which `isNull` 
returns false, but whose payloads are nevertheless `null`.