[swift-evolution] Enhanced existential types proposal discussion

[Brent Royal-Gordon]

> One initial bit of feedback -  I believe if you have existential types, I believe you can define Sequence Element directly, rather than with a type alias. e.g.
> 
> protocol Sequence {
>   associatedtype Element
>   associatedtype Iterator: any<IteratorProtocol where IteratorProtocol.Element==Element> 
>   associatedtype SubSequence: any<Sequence where Sequence.Element == Element>
>   …
> }

That's not really the same thing. Any<IteratorProtocol> is an existential, not a protocol. It's basically an automatically-generated version of our current `AnyIterator<T>` type (though with some additional flexibility). It can't appear on the right side of a `:`, any more than AnyIterator could.

What *would* work is allowing `where` clauses on associated types:

> protocol Sequence {
>   associatedtype Element
>   associatedtype Iterator: IteratorProtocol where Iterator.Element==Element
>   associatedtype SubSequence: Sequence where SubSequence.Element == Element
>   …
> }

I believe this is part of the generics manifesto.

-- 
Brent Royal-Gordon
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