UnionType.scala

sealed trait |[A, B] {
  def value: Any
  def as[T]: T = value.asInstanceOf[T]
}

object | {
  private case class Union[A, B](_1: Option[A], _2: Option[B]) extends |[A, B]{
    def value: Any = (_1 match {
      case Some(x: |[_, _]) => Some(x.value)
      case x => x
    }).getOrElse(_2.get)
  }

  implicit def A_to_Bar2[A, B](a: A): A | B = Union(Some(a), None)
  implicit def B_to_Bar2[A, B](a: B): A | B = Union(None, Some(a))
  implicit def A_to_Bar3[A, B, C](a: A): A | B | C = Union(Some(a), None)
  implicit def B_to_Bar3[A, B, C](a: B): A | B | C = Union(Some(a), None)
  implicit def A_to_Bar4[A, B, C, D](a: A): A | B | C | D = Union(Some(a), None)
  implicit def B_to_Bar4[A, B, C, D](a: B): A | B | C | D = Union(Some(a), None)
}

import |._

object UnionTypeTest extends App {
  def test(arg: String | Int | Boolean): Unit = {
    arg.value match {
      case x: String  => println("String")
      case x: Int     => println("Int")
      case x: Boolean => println("Boolean")
    }
  }

  test("str")
  test(123)
  test(true)
}