What does the following error message mean?

cannot override a concrete member without a third member that's overridden by both (this rule is designed to prevent ``accidental overrides'');

I was trying to do stackable trait modifications. It's a little bit after the fact since I already have a hierarchy in place and I'm trying to modify the behavior without having to rewrite a lot of code.

I have a base class called AbstractProcessor that defines an abstract method sort of like this:

class AbstractProcessor {
  def onPush(i:Info): Unit
}

I have a couple existing traits, to implement different onPush behaviors.

trait Pass1 {
  def onPush(i:Info): Unit = { ... }
}

trait Pass2 {
  def onPush(i:Info): Unit = { ... }
}

So that allows me to use new AbstractProcessor with Pass1 or new AbstractProcessor with Pass2.

Now I would like to do some processing before and after the onPush call in Pass1 and Pass2 while minimizing code changes to AbstractProcessor and Pass1 and Pass2. I thought of creating a trait that does something like this:

trait Custom extends AbstractProcessor {
  abstract override def onPush(i:Info): Unit = {
    // do stuff before
    super.onPush(i)
    // do stuff after
}

And using it with new AbstractProcessor with Pass1 with Custom and I got that error message.