The NaN of types (in D)

[Paul Backus]

On Sunday, 11 October 2020 at 12:18:59 UTC, Stefan Koch wrote:
> So we need a type that behaves just like Terror (i.e. is not 
> considered a type by the is expression) but is not an error.
>
> This type is the empty type (Ø).
> It is emptier than void and on the same level as bottom (⊥).
> But Ø cannot be ⊥, because ⊥ can be a valid return type of a 
> function.
> (At least once we have the ⊥ type)
>
> Ø not being a valid type as far as the is expression is 
> concerned.
> (is(Ø) yields false)
> Also means that is(Ø == Ø) also yields false.

To me, this sounds more like an empty Optional!__type or a null 
__type* than a NaN. Maybe it would be better to leave __type as 
the non-nullable version, and use something like __type* to 
represent a type that may be absent.