test whether implicit evidence needs both covariance and contravariance

exp.scala

object Crap extends App {
  sealed abstract class <=<[From, +To] extends (From => To) {
  }

  implicit def conforms[A]: <=<[A, A] = new <=<[A, A] {
    def apply(from: A): A = ???
  }

  class A[T] {
    def foo[U](u: U)(implicit ev: <=<[U, T]) = ()
    def bar[U](u: U)(implicit ev: <=<[T, U]) = ()
  }

  val a = new A[AnyVal]

  val x: Any = 12

  a.foo(12)
  a.bar(x)

}