Open Radar

 
Try the code below in a playground. np and dp are of identical formal types, but they dispatch differently, even though neither object has an actual doFoo() implementation of its own.

protocol Foo { func doFoo() }

protocol Qux {}

extension Foo {
    func doFoo() { print("Called default doFoo") }
}

extension Foo where Self: Qux {
    func doFoo() { print("Called Qux-specific doFoo") }
}

class BaseClass: Foo {}
class DerivedClass: BaseClass, Qux {}

class NativeFooQuxClass: Foo, Qux {}

let n = NativeFooQuxClass()
let d = DerivedClass()

n.doFoo() // Calls Qux-specific doFoo - correct
d.doFoo() // Calls Qux-specific doFoo - correct

let np: protocol<Foo, Qux> = n
let dp: protocol<Foo, Qux> = d

np.doFoo() // Calls Qux-specific doFoo - correct
dp.doFoo() // Calls default doFoo - incorrect