Scala: Type matching of a type alias with upper bound

Question

My question is simple.

Why does this code make an error,

abstract class A {
  type T <: A.Inner
  def toT: T = new A.Inner(this)
}

object A {
  class Inner(a: A)
}

// Exiting paste mode, now interpreting.

<console>:16: error: type mismatch;
 found   : A.Inner
 required: A.this.T
         def toT: T = new A.Inner(this)
                      ^

whereas this code does not?

abstract class A {
  type T = A.Inner
  def toT: T = new A.Inner(this)
}

object A {
  class Inner(a: A)
}

// Exiting paste mode, now interpreting.

defined class A
defined object A

A.Inner <: A.Inner. Isn't it?

score 2 · Accepted Answer · answered May 04 '16 at 06:32

Here, lower-bound should be used:

abstract class A {
  type T >: A.Inner
  def toT: T = new A.Inner(this)
}

object A {
  class Inner(a: A)
}

Only if T is ancestor of A.Inner, then A.Inner can be converted to T. We use lower-bound to restrict T is ancestor of A.Inner.

Scala: Type matching of a type alias with upper bound

1 Answers1