The problem with implicit classes.

gistfile1.scala

trait Height[P[_]] extends Any {
  def self: P[_]

  def height: Int
}

object Height {
  implicit class ListHeight(val self: List[_]) extends AnyVal with Height[List] {
    def height = self.size
  }
}

object Main extends App {
  import Height._
  
  List(1,2,3).height //works!
  
  def fn[T[_]: Height](t: T[_]) = t.height//doesn't
}

I wish they worked.