attempt at newtype in Scala for FKs

FK.scala

class FK[A] extends StaticAnnotation

class User
class Checkin

object Main {
  def fetch[A](id: Int @ FK[A]): A = null.asInstanceOf[A]

  def main(args: Array[String]): Unit = {
    val id: Int @ FK[Checkin] = 10

    // Why does this type check?
    fetch[User](id)
  }
}

Pretty hot, thanks! Miles' approach looks promising, but if it doesn't work out I'll give this a shot.