Unsoundness due to `erased`

Related to #50 #4042 #4031.

object App {
  def coerce[U, V](u: U): V = {
    trait X { type R >: U }
    trait Y { type R = V }

    class T[A <: X](ghost val a: A)(val value: a.R)

    object O { lazy val x : Y & X = ??? }

    val a = new T[Y & X](O.x)(u)
    a.value
  }

  def main(args: Array[String]): Unit = {
    val x: Int = coerce[String, Int]("a")
    println(x + 1)
  }
}

//////////////////////////////////////////////////////////////

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:101)
	at App$.main(HelloWorld.scala:15)
	at App.main(HelloWorld.scala)

[odersky]

Unfortunately, this makes erased vals practically useless. We should backout this fix and mark erased vals as unrealizable instead. erased is really the same as lazy in this respect. Either implies that the right-hand side will be constructed.

[Blaisorblade]

@alexknvl I renamed unused to ghost throughout the issue because of #4061.