SE-0353: Constrained Existential Types

[hborla (Holly Borla)]

@kemchenj

Dose any Collection<Int> conforms to Collection ? Or something like this valid:

func foo<C: Collection>(_ collection: C) { ... }

var bar: any Collection<Int> = ...
foo(bar)

any Collection<Int> does not conform to Collection (what would the Index and other associated types be?), however, the code you wrote is still valid thanks to existential opening/unboxing, which is under a second round of review now: SE-0352 (second review): Implicitly Opened Existentials