aux.scala

type |[A, B] = (A, B)

trait OneOfProof[Obj, Items]{
  type Value <: Bool
}

object OneOfProof extends LowPriorityOneOf{
  def apply[Obj, Items](implicit oneOf: OneOfProof[Obj, Items]): Aux[Obj, Items, oneOf.Value] = oneOf
  
  implicit def left[L, R, Obj <: L]: Aux[Obj, L | R, True] = new OneOfProof[Obj, L | R]{
    type Value = True
  }
}

trait LowPriorityOneOf extends LowPriorityOneOf1{
  implicit def right[L, R, Obj <: R]: Aux[Obj, L | R, True] = new OneOfProof[Obj, L | R]{
    type Value = True
  }
}

trait LowPriorityOneOf1 extends LowPriorityOneOf2{
  implicit def recur[L, R, Obj](implicit oneOf: OneOf[Obj, R]): Aux[Obj, L | R, oneOf.Value] =
    new OneOfProof[Obj, L | R]{
      type Value = oneOf.Value
    }
}

trait LowPriorityOneOf2{
  type Aux[O, I, B <: Bool] = OneOfProof[O, I]{ type Value = B }
  
  implicit def not[O, I]: Aux[O, I, False] = new OneOfProof[O, I]{ 
    type Value = False 
  }
}