Possible solution for GIST 3517707 on url https://gist.github.com/3517707 Uses abstract types and overrides the type in the subclass

MoreTest.scala

class BaseElement {
  def foo() { println("I like foo") }
}

class SubElement extends BaseElement {
  def bar() { println("I like bar") }
}

trait Base {
  //Abstract type T must be BaseElement or a subclass of BaseElement
  type T <: BaseElement

  var map = Map[Int,  T]()

  // want to provide default impl for Sub (without depending on SubElement)
  map.get(0).get.foo()

  // want to be able to mutate Base (Map is immutable for thread safety so need to replace it)
  map = map -- map.keys.filter(_>0)
}

class Sub extends Base {
  //overriding the type T seem to only work if the type specified as an override satisfies the requirement of
  // the parent type. Changing this to String will fail to compile.
  override type T = SubElement

  // want to be able to call methods on SubElement without having to upcast
  map.get(0).get.bar()
}