Scala Generics are Unsound

[scabug]

The following code

class A {
  def compareTo(o: Any): Int = 0
}
class B extends A with Comparable[B] {
  def compareTo(b: B): Int = 0
}
object C {
  def main(args: Array[String]): Unit = {
    println(new B().compareTo(new Object()))
  }
}

compiles without warnings. And it should be executed without any runtime error. However, ClassCastException occurs when the code is executed.

This problem is also known as an already fixed bug of Java Generics:

http://www.concurrentaffair.org/2006/10/10/java-generics-unsound/

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9912?orig=1
Reporter: Kota Mizushima (kmizushima)
Affected Versions: 2.11.8, 2.12.0-M5

[scabug]

@szeiger said:
I think this should not compile in the first place. From http://www.scala-lang.org/files/archive/spec/2.11/05-classes-and-objects.html#class-members :

{quote}It is also an error if a template contains two members (directly defined or inherited) with the same name and the same erased type.{quote}

While compareTo in B has a different erasure than compareTo in A, B also inherits (and overrides) compareTo from Comparable, which has the same erasure.

[scabug]

kenji yoshida said:
compile error in Scala 2.9.3 and 2.10.6

Welcome to Scala version 2.9.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_79).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

class A {
  def compareTo(o: Any): Int = 0
}
class B extends A with Comparable[B] {
  def compareTo(b: B): Int = 0
}
object C {
  def main(args: Array[String]): Unit = {
    println(new B().compareTo(new Object()))
  }
}

// Exiting paste mode, now interpreting.

<console>:10: error: name clash between inherited members:
method compareTo:(o: Any)Int in class A and
method compareTo:(x$1: B)Int in trait Comparable
have same type after erasure: (o: java.lang.Object)Int
       class B extends A with Comparable[B] {
             ^

Welcome to Scala version 2.10.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

class A {
  def compareTo(o: Any): Int = 0
}
class B extends A with Comparable[B] {
  def compareTo(b: B): Int = 0
}
object C {
  def main(args: Array[String]): Unit = {
    println(new B().compareTo(new Object()))
  }
}

// Exiting paste mode, now interpreting.

<console>:10: error: name clash between inherited members:
method compareTo:(o: Any)Int in class A and
method compareTo:(x$1: B)Int in trait Comparable
have same type after erasure: (o: Object)Int
       class B extends A with Comparable[B] {
             ^

[scabug]

@adriaanm said:
We should fix this, but it likely won't be possible in 2.12 due to compatibility constraints.