Cannot override a type with non-volatile upper bound

Question

I have a compiler error in scala and I don\'t know what does it refer to:
Assume these declarations:

trait Abstract {
  type MyType
}
trait AInner
trait         


        

Answer 1

What is wrong with this?

trait B extends Abstract with A {
  override type MyType <: BInner with AInner
}

In any realization of trait B, MyType will always be the same type as seen from trait A, so upper-bounding it by itself does not make any sense.

If in the upper piece of code, it bothers you that you'll have to rewrite trait B if you change the bound in trait A, use:

trait A extends Abstract{
  type ABound = AInner
  type MyType <: AInner
}
trait B extends Abstract with A {
  override type MyType <: BInner with ABound
}