Question

Because of an issue with package name aux under Windows, I am moving a helper class within the package hierarchy of my library from

de.sciss.scalainterpreter.aux

to

de.sciss.scalainterpreter

The class is private to the library, i.e. private[scalainterpreter] object Helper.

Now using the Typesafe Migration-Manager, obviously it reports that the change is not compatible:

Found 2 binary incompatibiities
===============================
 * class de.sciss.scalainterpreter.aux.Helper does not have a correspondent
   in new version
 * object de.sciss.scalainterpreter.aux.Helper does not have a correspondent
   in new version

But I suspect that if client code does not call into either object, the interfaces are still compatible, and thus I can use a minor version increase to indicate the change, and allow those two versions to be used interchangeably.

Correct?

Answer 1

It’s easy to see how inlining can break client code, since the inlined code essentially bleeds into the client interface. This example really asks for a linkage error; we can experiment and do things like javap | grep Helper, but at some level you have to let scalac do its job.

package lib {

  object Lib {
    //import util.Helper
    @inline def result = Helper.help
  }

  //package util {

  private [lib] object Helper {
    @inline def help = "Does this help?"
  }
//}
}

Sample innocently bystanding client:

package client

object Test {
  import lib.Lib
  def main(args: Array[String]) {
    println(Lib.result)
  }
}

Changing package of package-private class:

$ scala -cp "classes;target" client.Test
Does this help?

apm@halyard ~/tmp/taking-it-private
$ vi lib.scala

apm@halyard ~/tmp/taking-it-private
$ rm -rf classes/*

apm@halyard ~/tmp/taking-it-private
$ smalac -d classes -optimise lib.scala 

apm@halyard ~/tmp/taking-it-private
$ smala -cp "classes;target" client.Test
java.lang.ClassNotFoundException: lib.util.Helper$

Javap shows why. [Namely, the call is inlined but it still wants to init the module.]

I haven’t followed the discussions, but for example there are links at:
https://github.com/scala/scala/pull/1133
and other discussions on the ML about what expectations about binary compatibility are valid.
https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/sJ-xnWL_8PE