[swift-evolution] Enhanced existential types proposal discussion

[Austin Zheng]

> 
> I think that *all* methods should be available - at least in principle - with associated types 
> - replaced by their upper bounds (i.e. Any if no constraints have been given either by the protocol definition itself or th existential) if in covariant position and 
> - replaced by their lower bounds if in contravariant position
> 
> As it is not possible to define lower bounds in Swift, the lower bounds are always the bottom type (called `Nothing` in Swift and not be confused with optionals). The bottom type has no members and therefore a method referencing that type cannot be called and is effectively not available.
> 

Thanks for the feedback! So methods that have associated types in contravariant position would have those types be Nothing, unless there was a concrete type bound to that associated type in the Any's where clause?

Example

protocol MyProtocol {
  associatedtype AssocType1
  associatedtype AssocType2
  func foo(x: AssocType1, y: AssocType2)
}

let a : Any<MyProtocol>
// on 'a', foo is exposed as 'foo(x: Nothing, y: Nothing)', and can thus not be called

let b : Any<MyProtocol where .AssocType1 == Int>
// on 'b', foo is exposed as 'foo(x: Int, y: Nothing)' and still can't be called

let c : Any<MyProtocol where .AssocType1 == Int, .AssocType2 == String>
// on 'c', foo is exposed as 'foo(x: Int, y: String)', and can therefore be called

Let me know if this is what you had in mind.

Austin

> -Thorsten 

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