Why is the type inferred for f1 but not for f2?

RightToLeftInference.scala

implicit class Provided[A](f: A => A) {
  def provided(c: A => Boolean): A => A = 
    a => if (c(a)) f(a) else a
  
  def ->:(c: A => Boolean) = provided(c)
  def <--(c: A => Boolean) = provided(c)
}

def add(x: Int) = (_:Int) + x
val f1 = add(3) <-- (_ < 5)
val f2 = (_ < 5) ->: add(3)

// error: missing parameter type for expanded function ((x$1) => x$1.$less(5))
//  val f2 = (_ < 5) ->: add(3)
//            ^