Limits of implicit conversion of class arrays

[Dmitry Olshansky]

On Saturday, 23 March 2024 at 09:08:45 UTC, Per Nordlöw wrote:
> Is there a reason why
>
> ```d
> class Base {}
> class Derived : Base {}
>
> @safe pure nothrow unittest {
> 	Base b;
> 	Derived d;
> 	b = d; // pass
>
> 	Base[] bs;
> 	Derived[] ds;
> 	bs ~= ds; // pass
> 	bs = ds; // fail [1], should pass
> 	bs = cast(Base[])ds; // fail [2], should pass
> }
> ```
>
> fails as
>
> [1]: cannot implicitly convert expression `ds` of type 
> `Derived[]` to `Base[]`
> [2]: cast from `Derived[]` to `Base[]` not allowed in safe code
>
> ?

The first and second is unsound (infamously allowed in Java). 
Once you cast the slice you can populate it with Derived2 objects 
that are not Derived, hence breaking type safety of the ds slice.

—
Dmitry Olshansky
CEO @ Glow labs
https://olshansky.me