SE-0309: Unlock existential types for all protocols

[anthonylatsis]

@Paul_Cantrell

I wasn’t aware path-dependent types were on the table, even the far-future hypothetical table! It seemed from the responses to the perennial “optionals should be like Kotlin” question that Swift was steering clear of them for the foreseeable future. Interesting.

Your mention of Kotlin just made me realize that "path-dependent" might sound misleading to some. What we really mean is a notion similar to the type identity in opaque types rather than "smart casts":

@anandabits

protocol P {
  associatedtype B: Class
  var b: B { get }
  func takesB(_: B)
}

func takesP(p1: P, p2: P) {
 let b1 = p1.b 
 let b2 = p2.b
 p1.takesB(b1) // ok
 p2.takesB(b1) // error: b1 may be of the wrong type
 p2.takesB(b2) // ok
 p1.takesB(b2) // error: b2 may be of the wrong type

}