varargs does not works

Hi,
how can define function with  varargs ? 

b can not be A*, does works with List[A]. Any idea how to make it works? Thanks
@typeclass trait CanTruthy[A] { self =>
  /** Return true, if `a` is truthy. */
  def truthy(a: A, b: A*): Boolean
}
object CanTruthy {
  def fromTruthy[A](f: A => Boolean): CanTruthy[A] = new CanTruthy[A] {
    def truthy(a: A, b: A*): Boolean = f(a)
  }
}