How do we get over this hump?

gistfile1.scala

trait Height[T[_], U] {
  def self: T[U]
  def height: Int
  def getter(i: Int): U
}

object Height {
  implicit class ListHeight[T](val self: List[T]) extends Height[List,T] {
    def height = self.size
    def getter(i: Int) = self(i)
  }
}


// Doesn't compile
def fn[U,T[_] <% Height[T,U]](t: T[U]) = t.getter(5)