Calculate the Least Upper Bound of two types.

lub2.scala

object test {

  case class L[A, B]() {
    def ToLub[AA >: A <: L, BB >: B <: L, L] = new { type LUB = L }
  }

  val intBoolLub = L[Int, Boolean].ToLub
                             
  (1: AnyVal) : intBoolLub.LUB
  1: intBoolLub.LUB
  true: intBoolLub.LUB
  0L: intBoolLub.LUB


  // 
  // "": intBoolLub.LUB
}

Why doesn't this work?

object lub {

  case class L[A, B]() {
    def ToLub[AA >: A <: L, BB >: B <: L, L] = new { type LUB = L }
  }

  def apply[A,B] = L[A,B].ToLub

}

class Fruit
class Kiwi extends Fruit
class Apple extends Fruit

val kal = lub[Kiwi,Apple]

It gives

scala> :load lub.scala
Loading lub.scala...
defined module lub
defined class Fruit
defined class Kiwi
defined class Apple
kal: java.lang.Object{type LUB = Any} = lub$L$$anon$1@306f7492

The return type of apply is based on the inferred type arguments A=Any, B=Any, and L=Any, long before any fruit is passed it's way.

Is it possible to make it work without the slightly clumsy ToLub call (or rather, can you hide it)?

Same problem as here
akka/akka#19418