[swift-evolution] Enhanced existential types proposal discussion

[Brent Royal-Gordon]

>   // With the new collection model (SE-0065), indices are simply Comparable now
>   // Okay to cast from x.Index to Comparable
>   let a = anIndex as Comparable
> 
>   // Not okay to cast from Comparable to y.Index
>   let b = a as y.Index
> 
>   // This should be okay. We may want to break this feature into 'Opening Existentials' follow-up proposal
>   if let b = a as? y.Index {
>     // do stuff with b
>   }

If you're going to do this, it might make sense to add a new form of `as?`/`as!` cast for casting between the same associated type on different existentials:

	if let b = x.startIndex as? y.Index {
		// do stuff with b
	}

This would succeed if the concrete types of `x` and `y` had the same type for `Index`, and fail otherwise.

-- 
Brent Royal-Gordon
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