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

[Meta]

On Tuesday, 21 November 2017 at 09:12:25 UTC, Ola Fosheim Grostad 
wrote:
> On Tuesday, 21 November 2017 at 06:03:33 UTC, Meta wrote:
>> I'm not clear on whether he means that Java's type system is 
>> unsound, or that the type checking algorithm is unsound. From 
>> what I can tell, he's asserting the former but describing the 
>> latter.
>
> He claims that type systems with existential rules, 
> hierarchical relations between types and null can potentially 
> be unsound. His complaint is that if Java had been correctly 
> implemented to the letter of the spec then this issue could 
> have led to heap corruption if exploited by a malicious 
> programmer.
>
> Runtime checks are part of the type system though, so it isn't 
> unsound as implemented as generated JVM does runtime type 
> checks upon assignment.
>
> AFAIK the complaint assumes that information from generic 
> constraints isn't kept on a separate level.
>
> It is a worst case analysis of the spec...

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?