protocol P { associatedtype A }
class C : P { typealias A = Int }
func foo<T : C>(x: T) {
  let a: T.A? = nil
}

<stdin>:4:12: error: ambiguous type name 'A' in 'T'
  let a: T.A? = nil
         ~ ^
<stdin>:2:25: note: found candidate with type 'A' (aka 'Int')
class C : P { typealias A = Int }
                        ^
<stdin>:1:29: note: found candidate with type '<<error type>>'
protocol P { associatedtype A }
                            ^