Single level merge

merge.scala

trait Merge[LSet, RSet]{
  type Out
}

object Merge extends LowPriorityMerge{
  implicit def left[L, R]: Aux[L, L|R, L|R] = new Merge[L, L|R]{ type Out = L|R }
  
  implicit def right[L, R]: Aux[R, L|R, L|R] = new Merge[R, L|R]{ type Out = L|R }
}

trait LowPriorityMerge{
  type Aux[L, R, O] = Merge[L, R]{ type Out = O }
  
  implicit def none[S, T]: Aux[S, T, S|T] = new Merge[S, T]{ type Out = S|T }
}