Adding method to value class breaks binary compatibility but is not reported

I made the following change. From

  implicit class Ops(private val s: String) extends scala.AnyVal {
    def `\\` (index: Int  ): String = s.substring(index, index + 1)
  }

to

  implicit class Ops(private val s: String) extends scala.AnyVal {
    def `\\` (index: Int  ): String = s.substring(index, index + 1)
    def `\\` (range: Range): String = range.map(s.charAt).mkString
  }

Now here are multiple things that could be involved:

• it’s a value class (my guess)
• it’s an overloaded method
• it’s an implicit class

I have created a project that demonstrates the bug: https://github.com/Sciss/mima-bug-135 To reproduce:

• git clone https://github.com/Sciss/mima-bug-135.git
• cd mima-bug-135
• git checkout foo-0.1.0
• cd foo && sbt publish-local
• cd ../bar && sbt publish-local
• cd ../baz && sbt run

This should run without crash. Now we publish the updated version:

• git checkout foo-0.1.1
• cd ../foo && sbt mimaReportBinaryIssues && sbt publish-local
• cd ../baz && sbt run

(note that Bar was compiled against foo-0.1.0; MiMa does not report any issue).

This crashes with:

error] (run-main-0) java.lang.NoSuchMethodError: de.sciss.mima.package$Ops$.$bslash$extension(Ljava/lang/String;I)Ljava/lang/String;
java.lang.NoSuchMethodError: de.sciss.mima.package$Ops$.$bslash$extension(Ljava/lang/String;I)Ljava/lang/String;
	at de.sciss.mima.Bar$.apply(Bar.scala:4)
	at de.sciss.mima.Baz$.main(Baz.scala:4)
	at de.sciss.mima.Baz.main(Baz.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)

[dwijnand]

It’s not clear to me why such a change should not be binary compatible.

Fixed in 2.13.0-RC1 with https://github.com/scala/scala/pull/7896

cc also @kailuowang

[szeiger]

The problem here is that any member whose decoded name contains a $ is considered non-accessible. I think the most straight-forward fix would be to exclude names that end in $extensions or that contain $extension$. It feels a bit dirty but so does excluding every name with a $ in the first place.