Unboxed newtype in Scala (with sugar)

gistfile1.scala

class User
class Checkin
  
type Tagged[T] = { type Tag = T }
type @@[T, U] = T with Tagged[U]
  
def tag[T] = new {
  def apply[U](u : U) : U @@ T = u.asInstanceOf[U @@ T]
}

def fetch[A](id: Int @@ A): A = null.asInstanceOf[A]

def main(args: Array[String]): Unit = {
  val id = tag[Checkin](10)
    
  fetch[Checkin](id) // Compiles
  // fetch[User](id)    // Does not compile
    
  val ids = tag[User](1) :: tag[User](2) :: tag[User](3) :: Nil
    
  val users : List[Int @@ User] = ids         // Compiles
  // val checkins : List[(Int with Tagged[Checkins])] = ids  // Does not compile
}

Maybe it doesn't compile because there is no "Checkins" type. (Ha ha.)

@paulp Oops ... corrected in the original (it still doesn't compile ;-).