Open Radar

 
Summary:
I was trying to implement double-dispatching for which I need correct type preservation. But if I'm right this is broken as my dispatch generic function doesn't preserve enough of the type information to perform correct overloading.

Steps to Reproduce:
Paste the following in a playground:

class Base {}
class Derived : Base {}

class Dispatcher {
    
    func dispatch<T: Base>(t: T) {
        // At this point compiler knows that t is an instance of T which *might* be Base or derived from Base.
        // Yet its overload resolution always treats is as Base not as T
        accept(t)
    }
    
    func accept(t: Base) {
        println("accept(Base)")
    }
    func accept(t: Derived) {
        println("accept(Derived)")
    }
    
}

var base = Base()
var derived = Derived()
var dispatcher = Dispatcher()
dispatcher.dispatch(base)
dispatcher.dispatch(derived)


Expected Results:
accept(Base)
accept(Derived)

Actual Results:
accept(Base)
accept(Base)

Version:
Xcode Version 6.0 (6A215l)
OS X 10.9.3 (13D65)


Notes:
I haven't figured out any workaround. I tried explicitly casting "t" to "T" ("t as T") but it didn't help.