Introducing Nullable Reference Types in C#. Is there hope for D, too?

[Ola Fosheim Grøstad]

On Tuesday, 21 November 2017 at 18:00:37 UTC, Meta wrote:
> I don't quite understand the logic here, because it seems to be 
> backwards reasoning. Constrain<U,? super T> is a valid type 
> because null inhabits it? That doesn't make sense to me. He 
> also cites the "implicit constraint" that X extends U where X 
> is ? super T, but X does not meet that constraint (Constrain<U, 
> X extends U>) so how can  the type checker deduce that X 
> extends U?

I haven't dug into the details of the paper as I think the 
authors didn't try to appear neutral, e.g. quoting null as the 
billon dollar mistake, and made their finding seem more 
spectacular than it is… What I get from skimming over it is this:

You get a call:

     upcast(constrain,x) -> String

    where:
        constrain is of type Constrain<String, X>
        x is of type X  (X is unspecified supertype of Integer)
        return type is String, which has X as subclass

    So you get String <: X <: Integer

The deduction that String is a superclass of Integer could come 
from:
     Constrain<String, X>

    where X = ? super Integer = unknown type that is supertype of 
Integer