Type inference of curried vs uncurried fold on Option

folds.scala

object CurryInference {
  implicit final class OptionOps[A](val oa: Option[A]) {

    /** curried fold */
    def curriedFold[B](z: => B)(f: A => B): B = oa.fold(z)(f)

    /** uncurried fold */
    def uncurriedFold[B](z: => B, f: A => B): B = oa.fold(z)(f)
  }

  val age: Option[Int] = Some(19)

  /*
   * error: type mismatch;
   *  found   : scala.util.Right[Nothing,Int]
   *  required: scala.util.Left[String,Nothing]
   *   age.curriedFold(Left("age is required"))(i => if (i >= 0) Right(i) else Left(s"$i is not positive"))
   *                                                            ^
   */
  //age.curriedFold(Left("age is required"))(i => if (i >= 0) Right(i) else Left(s"$i is not positive"))

  /*
   * Compiles. Without currying, LUB is determined to be Either[String, Int]
   *
   * Technically the LUB probably also has Product with Serializable
   */
  age.uncurriedFold(Left("age is required"), i => if (i >= 0) Right(i) else Left(s"$i is not positive"))
}