Add test for public method becomes private[lib]

[ashawley]

This came up as an issue when updating to MiMa 0.5.0 in ScalaCheck, see typelevel/scalacheck#509

[dwijnand]

It passed, so this isn't an accurate minimisation.

[ashawley]

There didn't appear to be a test for making a method package-private, so I wanted to verify the base case. Although, I tried to add the complexity of implicits and overloading that were in the issue from ScalaCheck. I wonder what I'm missing to reproduce this? I'll try some more...

[ashawley]

Here's the test failure, but it's not the same one...

[dwijnand]

Yeah that the loss of a static forwarder, which is a potential bincompat concern.

[ashawley]

A simple test suggests that marking a static method as package-private should be binary compatible:

package bar
class A[T](t: T)
object A {
  def foo[T](x: T): A[T] = new A(x)
  def foo[T](x: T, y: T): A[T] = new A(y)
}

package bar
class A[T](t: T)
object A {
  def foo[T]( x: T): A[T] = new A(x)
  private[bar] def foo[T](x: T, y: T): A[T] = new A(y)
}

$ sbt test-public-method-becomes-package-private-ok/test
$ cd functional-tests/src/test/public-method-becomes-package-private-nok
$ scalac -classpath $(echo target/scala-2.12/*-V1.jar) -d ./classes App.scala
$ scala -classpath $(echo target/scala-2.12/*-V1.jar):./classes baz.App
Success
$ scala -classpath $(echo target/scala-2.12/*-V2.jar):./classes baz.App
Success

Here's baz.App:

package baz
object App {
  def main(args: Array[String]): Unit = {
    bar.A.foo[Int](1,2)
    println("Success")
  }
}

Is this a false positive?

[dwijnand]

No. Scala doesn't use the static method, it uses the non-static method on the singleton object value. The binary incompatibility is only observable at runtime by calling from Java.

[ashawley]

I've converted the example App in to Java. Assuming I'm correctly calling the static method from Java as you suggested, it still doesn't seem to throw an exception for me:

$ javac -cp ./target/scala-2.12/*-V1.jar -d ./classes App.java
$ java -cp ~/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.9.jar:$(echo target/scala-2.12/*-V1.jar):./classes baz.App
Success
$ java -cp ~/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.9.jar:$(echo target/scala-2.12/*-V2.jar):./classes baz.App
Success

package baz;
class App {
  public static void main(String[] args) {
    bar.A$.MODULE$.foo(1,2);
    System.out.println("Success");
  }
}

[dwijnand]

Nope. MODULE$ is the singleton instance. The static method is bar.A.foo, the same syntax you had in Scala.

[ashawley]

Ok, now I follow. Yes, indeed. It fails with that single line change:

package baz;
class App {
  public static void main(String[] args) {
    bar.A.foo(1,2); // !!!
    System.out.println("Success");
  }
}

Resulting in:

Exception in thread "main" java.lang.NoClassDefFoundError: bar/A
        at baz.App.main(App.java:4)
Caused by: java.lang.ClassNotFoundException: bar.A
        at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 1 more

Thanks for explaining this! Sorry for all the questions.