Confusing opening existentials syntax

[Karl]

I suggested the following syntax in the opaque types manifesto:

@Karl

var objects: Collection = ...
var openedObjects: <X: Collection> = objects

var destination: Collection = ...
if var rrc = destination as? <R> where R: RangeReplaceableCollection, R.Element == X.Element {
  rrc.append(contentsOf: openedObjects)
  destination = rrc
}