Imported From: https://issues.scala-lang.org/browse/SI-8899?orig=1
Reporter: Kazuhiro Sera (seratch)
Affected Versions: 2.11.3

@retronym said:
Lukas, looks like this stems from 9276a1205f74fdec74206209712831913e93f359. Did we reason incorrectly about the binary compatibilty of that change?

kenji yoshida said:
more simple example

https://github.com/xuwei-k/SI-8899

Takashi Kawachi (tkawachi) said:
As kenji shown in the sample code, it's not specific to Scalatra. It seems to be caused when it extends standard collection.

kenji yoshida said (edited on Oct 12, 2014 10:39:30 AM UTC):
another examples

• https://gist.github.com/xuwei-k/5028ccf9a4d4af890d0f
• https://gist.github.com/xuwei-k/aa558ab4b5729394a33b

kenji yoshida said:
also https://github.com/playframework/playframework/blob/2.3.5/framework/src/play/src/main/scala/play/api/mvc/Http.scala#L683

@gkossakowski said:
The related PR is: scala/scala#3949

@lrytz said (edited on Oct 12, 2014 4:58:56 PM UTC):
Making filterImpl non-private in PR #3949 was indeed not binary compatible. The problem is that making a trait method non-private changes its calling convention. Here's an example:

trait Tl {
  private def f1Impl = 0
  def f1 = f1Impl

  private[p] def f2Impl = 0
  def f2 = f2Impl
}

class Mp extends Tl

The point is that the invocation of f1Impl is statically known. So the compiler optimizes it and directly invokes the static method f1Impl in the trait implementation class Tl$class.

Method f2Impl on the other hand may be overwritten in subclasses of Tl, therefore the invocation is compiled as an invoke-interface of the method f2Impl in interface Tl. In the subclass Mp, a mixin is generated for f2Impl.

public abstract interface p/Tl {
  // NOTE: f1Impl is missing - it's private!
  public abstract f1()I
  public abstract f2Impl()I
  public abstract f2()I
}

public abstract class p/Tl$class {
  private static f1Impl(Lp/Tl;)I
  public static f1(Lp/Tl;)I
    ALOAD 0
    INVOKESTATIC p/Tl$class.f1Impl (Lp/Tl;)I  // Directly invokes the static method

  public static f2Impl(Lp/Tl;)I
  public static f2(Lp/Tl;)I
    ALOAD 0
    INVOKEINTERFACE p/Tl.f2Impl ()I  // Interface method
}

public class p/Mp implements p/Tl  {
  // generated mixins (forward to the static functions in Tl$class)
  public f1()I
  public f2Impl()I
  public f2()I
}

The manifestation: subclasses of TraversableLike compiled with 2.11.2 don't have a mixin for filterImpl, but the standard library of 2.11.3 invokes the interface method.

@lrytz said:
scala/scala#4048

@Blaisorblade said:
If anybody is wondering (like me) why MIMA didn't catch it, don't worry, MIMA is fine:
lrytz/scala@9276a12

The corresponding mima-failures can be whitelisted, as the changes are only to private[scala].