A design question when using stackable trait

Hi list -
being new to Scala, I have a design question when using stackable trait.
I have a class here -

class Executor [T <: Task] {
  def add(task: T) = {
...
  }
}

And then, I'd like to have a 'wrapper' on this, to do with logging of task executing.
Something like this -

trait ExeLoger[T <: Task ] extends Executor[T] {
  override abstract def add(task: T) = {
    println shower.show(task) + "added"// I want to add a shower for task here
    super.add(task)

  }
}

As the comment on the println line, I want a class type for show task here.
So I modified the trait like this -

trait ExeLoger[T <: Task : Descriptor ] extends Executor[T]

But this doesnt compile. Saying that 'traits cannot have type parameters with context bounds `: ...' nor view bounds `<% ...''
So, gurus, could please give me some advise? How to design to solve this problem?
Is this a right place to use stackable trait?