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)
  }
}

Miles' approach might have obviated the need for annotation checking, but I think you can add extra typechecking with a custom AnnotationChecker.

AnnotationCheckers:

/** Register an annotation checker.  Typically these
   *  are added by compiler plugins. */
  def addAnnotationChecker(checker: AnnotationChecker) {
    if (!(annotationCheckers contains checker))
      annotationCheckers = checker :: annotationCheckers
  }

Typers:662

• (-1) For expressions with annotated types, let AnnotationCheckers decide what to do

Types:4458

tp1 match {
      case _: AnnotatedType =>
        return annotationsConform(tp1, tp2) && annotationsConform(tp2, tp1) && tp1.withoutAnnotations =:= tp2.withoutAnnotations
      case _ =>
    }

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