reimplementing an interface in a derived class

[Alex]

On Friday, 4 January 2019 at 07:37:43 UTC, bauss wrote:
> No, because you OVERRIDE A's foo().
>
> A does not exist. A is B and when you cast B to A you just tell 
> the compiler that the reference should only have A's signature 
> available.
>
> You're not assigning B to A.

Let's assume this is right. How to force a B object to behave 
like an A object? I thought casting is a possible approach...