-Xgen-mixin-forwarders crashes for default methods

[lrytz]

trait O[T] extends java.util.Comparator[T] // Comparator has a method `default Comparator<T> reversed`

class C extends O[String] {
  def compare(a: String, b: String) = 0
}

crashes the compiler with -Xgen-mixin-forwarders:

java.lang.AssertionError: assertion failed:
  cannot invokespecial java/util/Comparator.reversed, the interface is not a direct parent.

        at scala.tools.nsc.Global.assert(Global.scala:229)
        at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genCallMethod(BCodeBodyBuilder.scala:1089)
        at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genApply(BCodeBodyBuilder.scala:584)
        at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:298)
        at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.emitNormalMethodBody$1(BCodeSkelBuilder.scala:604)
        at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:636)

[lrytz]

maybe something to fix for RC1..?

[lrytz]

For reference: in M4 we add the interface as a direct parent during code gen when emitting a super call to a default method: https://github.com/scala/scala/blob/v2.12.0-M4/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala#L1083-L1084

In a later commit this information ("late interfaces") was also encoded in the ClassBType: scala/scala@3fca034

All of this was removed (for M5) when we changed the encoding to emit static methods for trait methods, super calls would directly target the static method (scala/scala@7d51b3f). A super call to a java-defined default method is now only allowed if the interface is a direct parent, this is checked during Mixins (scala/scala@7d51b3f#diff-8b2a191e6261b31a9a6462e4a01d3b9cR351).

The -Xgen-mixin-forwarders flag was added in (scala/scala@a0590aa), and it does its job also for inherited default methods. If the interface is not a direct parent of the class, an assertion in the backend will trigger the crash.

[lrytz]

Here's an example that crashes without the -Xgen-mixin-forwarders flag:

A.java:

interface A {
  default int m() { return 1; }
}

Test.scala

class B extends A { override def m = 2 }
trait T extends A
class C extends B with T {
  override def m = super[T].m // should invoke A.m
}

The super[T].m invocation is translated to invokespecial A.m. This causes the assertion error in the backend: A is not a direct parent of C.

To translate the static binding super[T].m, the compiler checks the owner of T.m, which is A. If A was a trait (written in Scala), the call would be translated to invokestatic A.m$. But the static method doesn't exist (A is written in Java), so the call is translated to invokespecial A.m.

This example is the same as in #143 (comment) but with of a Java-defined interface A instead of a superclass A. In this case, there also isn't a static method A.m$, so the call is translated to invokespecial A.m, which does not crash the backend (A is a class, the invokespecial is valid), but will resolve to B.m at runtime.

[lrytz]

scala/scala#5369