SE-0352: Implicitly Opened Existentials

[Douglas_Gregor]

@ensan-hcl

First, is cannotOpen(p1, p1) possible? I think it should not be possible, but I'd like to check my understanding.

func cannotOpen<T: P>(_ a: T, _ b: T) { ... }

func testCannotOpenMultiple(p1: any P) {
  cannotOpen(p1, p1)
}

No, opening is not possible here, because you wouldn't get a consistent T: each place where we opened an existential effectively creates a type that is unique to that opening operation.

@ensan-hcl

func foo(value: any P) {
    // here, what is the type of `a`?
    // if the result is type-erased to its upper bounds, it should be any Collection<.Index == Int>, but how the constraint can be expressed?
    let a = getA(value)
}

The "upper bound" here is currently defined to be any Collection, because the language can't currently express anything tighter than that. If existentials grow the ability to handle constraints (e.g., the constrained existentials from this pitch covers most of the cases, although not your specific one), the upper bound could be tightened.

Doug