subclassing a java annotation generates an invalid classfile

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JavaAnnot.java:

public @interface JavaAnnot { }

Test.scala:

class C extends JavaAnnot {
  def annotationType: Class[_ <: java.lang.annotation.Annotation] = null
}

object Test extends App {
  new C
}

There is also a (mostly) spurious warning emitted:

➜  sandbox git:(t9393) ✗ javac JavaAnnot.java
➜  sandbox git:(t9393) ✗ scalac Test.scala
Test.scala:1: warning: Implementation restriction: subclassing Classfile does not
make your annotation visible at runtime.  If that is what
you want, you must write the annotation class in Java.
class C extends JavaAnnot {
      ^
one warning found
➜  sandbox git:(t9393) ✗ scala Test
java.lang.IncompatibleClassChangeError: class C has interface JavaAnnot as super class
	at java.lang.ClassLoader.defineClass1(Native Method)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9400?orig=1
Reporter: @lrytz
Affected Versions: 2.11.7
See #8778, #9393

[scabug]

@lrytz said:
the reason is that the classfile / java source parser make class symbols for java annotations look like classes, not interfaces (the INTERFACE / DEFERRED flags are deliberately not added).

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@lrytz said:
fixing this problem (which I did locally) still produces an invalid classfile:

➜  sandbox git:(t9393) ✗ qs Test
java.lang.VerifyError: Bad <init> method call
Exception Details:
  Location:
    C.<init>()V @1: invokespecial
  Reason:
    Type 'JavaAnnot' is not assignable to 'C'
  Bytecode:
    0x0000000: 2ab7 0012 b1

	at Test$.<init>(Test.scala:6)
	at Test$.<clinit>(Test.scala)

[scabug]

@lrytz said (edited on Jul 15, 2015 7:26:40 PM UTC):
Yet another related issue: the classfile parser adds scala.annotation.Annotation as superclass for Java annotations, so the following compiles, but generates invalid bytecode:

object Test extends App {
  def foo(a: JavaAnnot): scala.annotation.Annotation = a
}

➜  sandbox git:(t9393) ✗ scalac Test.scala
➜  sandbox git:(t9393) ✗ scala Test
java.lang.VerifyError: Bad return type
Exception Details:
  Location:
    Test$.foo(LJavaAnnot;)Lscala/annotation/Annotation; @1: areturn
  Reason:
    Type 'JavaAnnot' (current frame, stack[0]) is not assignable to 'scala/annotation/Annotation' (from method signature)
  Current Frame:
    bci: @1
    flags: { }
    locals: { 'Test$', 'JavaAnnot' }
    stack: { 'JavaAnnot' }
  Bytecode:
    0x0000000: 2bb0

	at Test.main(Test.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

GenBCode catches the problem at bytecode generation time and crashes with an assertion error.

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@lrytz said:
related to scala/scala#4566, which adds a workaround for a similar hack for ENUM classfiles

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@lrytz said:
see also discussion on scala/scala#4638

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@lrytz said:
and also https://groups.google.com/forum/#!topic/scala-language/4Cjr_n3CAr0