General problem I'm having in D with the type system

[JN]

On Sunday, 27 May 2018 at 06:00:30 UTC, IntegratedDimensions 
wrote:
> animal a = new cat();
>
> a.f = new food()
> auto c = cast(cat)a;
>
>
> as now f in cat will be food rather than catfood.

I think the problem is in your hierarchy. If Animal can have 
Food, that means that any animal should be able to accept any 
food, without knowing what kind of food is this. Cat requiring 
cat food is a leaky abstraction, the cat shouldn't know nor care 
what kind of food it gets, as it's an animal and it will eat any 
food.