Covariant callback functions, or assigning base class members through a subclass reference

[Rene Zwanenburg via Digitalmars-d-learn]

On Tuesday, 14 July 2015 at 15:35:04 UTC, Kagamin wrote:
> Template Base on Derived:
>
> class Derived : Base!Derived
> {
>
> }

Sure, but that would make Base!Derived and Base!AnotherSubClass 
different types.

What I'd like to end up with is a Base[], being able to call 
foo() on the array members. Other parts of the code will add 
instances of derived types to this array, and have set the 
callback function to a delegate accepting said derived type.

This is part of a library I'm working on. I don't mind if the 
library itself contains some nastiness to get this to work, as 
long as user code, which defines the derived types, is type safe 
and clean.