trait A {
  type T
  var v : T
}

object B {
  def f(x : { val y : A }) { x.y.v = x.y.v } 
  
  var a : A = _
  var b : Boolean = false
  def y : A = {
    if(b) {
      a = new A { type T = Int; var v = 1 }
      a
    } else {
      a = new A { type T = String; var v = "" }
      b = true
      a
    }
  }
}

scala> B.f(B)
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
        at B$$$$anon$$2.v_$$eq(<console>:15)
        at B$$.f(<console>:6)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at RequestResult$$.<init>(<console>:4)
        at RequestResult$$.<clinit>(<console>)
        at RequestResult$$result(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)