Feature: `static cast`

[Quirin Schroll]

On Thursday, 1 June 2023 at 14:55:32 UTC, Richard (Rikki) Andrew 
Cattermole wrote:
> Alternatively we could just make down casts (including 
> truncation) without a null/size check unsafe.

Making down-casts without `null` checks unsafe wouldn’t solve the 
issue that `null` checks on `extern(C++)` class object down-casts 
don’t work properly. That would make the problem worse.