No polymorphism?

[Adam D. Ruppe via Digitalmars-d-learn]

On Monday, 24 July 2017 at 17:29:55 UTC, Dgame wrote:
> 	S[] ss = [new S()];
> 	test1(ss); // Fails
>
> Why isn't the compiler able to deduce S[] => I[]? Or is it just 
> me?

This is exactly because of polymorphism. Consider the following:

```
S[] ss = [new S()];
I[] i = ss; // pass it to the function or whatever for implicit 
conversion

class OtherDerived : I {}

i[0] = new OtherDerived(); // looks OK, otherDerived is also 
interface I
```

But now, ss[0], the same array as i, no longer points to an S! 
You broke the type system.