Type mismatch, expected: T, actual: T

Question

I'm not understanding what this error wants me to do:

Type mismatch, expected: T, actual: T

I only have 3 lines of code:

case class BaseElem[T](e: T)
case class OrderedElem[T <: Ordered](override val e: T) extends BaseElem[T](e) with Ordered[OrderedElem[T]] {
  override def compare(that: OrderedElem[T]): Int = this.e.compare(that.e)
}

BaseElem is a simple container of T's. OrderedElem is an Ordered container of T <: Ordered. So I want a comparison between OrderedElems to be the comparison of their respective elements.

The error is in the override of compare, the code highlighted by the error is that.e.

What is this error saying and how do I fix it?
Side question, can the declaration of OrderedElem be simplified and retain the desired semantics?

Some code smell in your snippet 1) [case classes inheritance is something you should avoid](https://stackoverflow.com/questions/11158929/what-is-so-wrong-with-case-class-inheritance), in fact there will be compiler error for recent scala versions 2) Naming **different** type aliases, in **different** classes with the same `T` letter is the source of errors you had at least 25 other options, pick, say `U` for the alias in `OrderedElem`. Finally, there is also `error: type Ordered takes type parameters` error — om-nom-nom, May 23 '14 at 20:03

score 1 · Accepted Answer · edited May 23 '17 at 12:01

The issue was with the part OrderedElem[T <: Ordered]. After following om-nom-nom's smell cleanups and fixing the error he noted, I found that types like Int (with primitive representations) don't extent Ordered in Scala (see this question), so one must use a "view bound" <% to tell Scala to also look for available implicit conversions.

Now I have:

class BaseElem[T](val e: T)

class OrderedElem[U <% Ordered[U]](override val e: U) extends BaseElem(e) with Ordered[OrderedElem[U]] {
  override def compare(that: OrderedElem[U]): Int = this.e.compare(that.e)
}

object Run extends App {
  val a = new OrderedElem(0)
  val b = new OrderedElem(1)
  println(a < b)  // => true
}

Type mismatch, expected: T, actual: T

1 Answers1