Using an extra object to infer one of two type parameters of a method

ToTest.scala

object ToTest extends App {

  val i1 = new Invoker(List(1,2,3,1))
  val t1 = i1.to[Set]()
  val t1t: Set[Int] = t1
  println(t1)
  
  val i2 = new Invoker(Seq("a","b"))
  val t2 = i2.to[Array]()
  val t2t: Array[String] = t2
  println(t2)
}

class Invoker[+R](val contents: Traversable[R]) {
  import scala.collection.generic.CanBuildFrom

  final def build[To](implicit canBuildFrom: CanBuildFrom[Nothing, R, To]): To = {
    val b = canBuildFrom()
    contents.foreach { x => b += x }
    b.result()
  }

  final def to[C[_]] = new To[C]()

  final class To[C[_]] private[Invoker] () {
    def apply[RR >: R]()(implicit canBuildFrom: CanBuildFrom[Nothing, RR, C[RR]]) =
      build[C[RR]](canBuildFrom)
  }
}