unsoundForward.scala

object unsoundForwardRef {
  trait LowerBound[T] {
    type M >: T;
  }
  trait UpperBound[U] {
    type M <: U;
  }
 
  val bounded1 : LowerBound[Int] with UpperBound[String] = hideForwardRef
  
  val bounded2 : LowerBound[Int] with UpperBound[String] = bounded1
  
  def hideForwardRef = bounded2
  
  
  def upcast[T](ub : LowerBound[T], t : T) : ub.M = t
  
  def main(args : Array[String]) : Unit = {
    val zero : String = upcast(bounded1, 0)
    println("...")
  }
}

unsoundStatics.scala

object unsoundStatics {
  trait LowerBound[T] {
    type M >: T;
  }
  trait UpperBound[U] {
    type M <: U;
  }
  object Coerce1 {
    val bounded : LowerBound[Int] with UpperBound[String] = Coerce1.bounded
  }
  object Coerce2 {
    val bounded : LowerBound[Int] with UpperBound[String] = Coerce2.bounded
  }
  
  def upcast[T](ub : LowerBound[T], t : T) : ub.M = t
  
  def main(args : Array[String]) : Unit = {
    val zero : String = upcast(Coerce1.bounded, 0)
    println("...")
  }
}