Cake pattern with overriding abstract type don’t work with Upper Type Bounds

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It’s a shortcoming of Scala’s type system. When determining the members in a mixin, Scala uses two rules: First, concrete always overrides abstract. Second, If two members are both concrete, or both abstract, then the one that comes later in linearization order wins.

Furthermore, the self type of a trait

trait S { this: C => ... }

is implicitly augmented to 

trait S { this: S with C => ... }

so that definitions in the trait S can be accessed within S. In your case, the trait S is seen as:

trait S { this: S with Cake =>
  type T = A
  def t: T
  t.ping
}

Now, as long as T is concrete this is fine because it overrides the abstract T in Cake. But if T is abstract, the one in Cake cames later in the linearization order and wins. And that T does not have an upper bound, so no member ping. One way to fix this is to change the linearization order by writing:

trait S { this: Cake with S =>
  type T <: A
  def t: T
  t.ping
}

It would be cleaner if Scala had a different rule that says that all constraints of abstract type members are merged in the mixin, instead of picking a single member according to linearization order. That’s a change we want to consider in the future, but we need to be careful with backwards compatibility.

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