Tped actor

Welcome to Scala.next (pre-alpha, git-hash: unknown)  (OpenJDK 64-Bit Server VM, Java 1.8.0_131).
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scala> trait ActorRef[T]
defined trait ActorRef
scala> case class Message(s: String, sender: ActorRef[Int])
defined class Message
scala> class Fut[T]()
defined class Fut
scala> trait Actor[T] { def ?[R](f: ActorRef[R] => T): Fut[R] = new Fut }
defined trait Actor
scala> object actor extends Actor[Message]
defined module actor
scala> val res: Fut[Int] = actor ? (Message("str", _))
val res: Fut[Int] = Fut@625dfff3
scala> actor ? (Message("str", _))
-- [E007] Type Mismatch Error: <console>:11:24 ---------------------------------
11 |actor ? (Message("str", _))
   |                        ^
   |                        found:    ActorRef[Any](_$1)
   |                        required: ActorRef[Int]
   |