Limits of implicit conversion of class arrays

[Per Nordlöw]

On Saturday, 23 March 2024 at 11:04:04 UTC, Dmitry Olshansky 
wrote:
> The first and second is unsound (infamously allowed in Java).

In the general case, yes. But, do you see any errors with the code

```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
}
```

> Once you cast the slice you can populate it with Derived2 
> objects that are not Derived, hence breaking type safety of the 
> ds slice.

Again, in the general case, yes.

So what is different in this code example compared to the general 
case? Hint: this has overlaps with a missing compiler 
optimization in dmd (and many other statically typed languages) 
enabled by a specific kind of data flow analysis. Which one?