foo-v1.scala

package object foo {
  class FooOps[A](self: A) {
    def foo: Int = self.##
  }
  implicit def toFooOps[A](a: A): FooOps[A] = new FooOps[A](a)
}

foo-v2.scala

package object foo {
  class FooOps[A](val self: A) extends AnyVal {
    def foo: Int = self.##
  }
  implicit def toFooOps[A](a: A): FooOps[A] = new FooOps[A](a)
}

gistfile1.txt

scalac foo-v1.scala
scalac -cp . MyApp.scala
scala -cp . MyApp
scalac foo-v2.scala
scala -cp . MyApp
java.lang.NoSuchMethodError: foo.package$.toFooOps(Ljava/lang/Object;)Lfoo/package$FooOps;
        at MyApp$.delayedEndpoint$MyApp$1(bar.scala:4)
        at MyApp$delayedInit$body.apply(bar.scala:3)
        at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
        at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
        at scala.App$$anonfun$main$1.apply(App.scala:76)
        at scala.App$$anonfun$main$1.apply(App.scala:76)
        at scala.collection.immutable.List.foreach(List.scala:381)
        at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
        at scala.App$class.main(App.scala:76)
        at MyApp$.main(bar.scala:3)
        at MyApp.main(bar.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

MyApp.scala

import foo._

object MyApp extends App {
  println(this.foo)
}