FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux lotp-laptop 2.6.20-15-generic #2 SMP Sun Apr 15 07:36:31 UTC 2007 i686 GNU

A DESCRIPTION OF THE PROBLEM :
In some cases javac outputs errors about inconvertible types. The circumstances in which this happens seem to be rather complicated. Trying to reduce the problem to a canonical form eradicates it.

Full discussion can be found at http://forum.java.sun.com/thread.jspa?threadID=5162196&tstart=0

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Try to compile the supplied source code example.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should compile without errors
ACTUAL -
javac outputs the following errors:

javac Bug.java
  Bug.java:27: inconvertible types
found   : JETEvent<capture#608 of ?>
required: FSDEvent
                if(event == null || ! (event instanceof FSDEvent)) { // <-- doesn't compile
                                       ^
  Bug.java:30: inconvertible types
found   : JETEvent<capture#870 of ?>
required: FSDEvent
                FSDEvent fsdEvent = (FSDEvent)event;
                                              ^
2 errors

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

abstract class JETEvent<E extends Enum<E>> {}
 
class FSDEvent extends JETEvent<FSDEvent.SUBTYPE> {
    public enum SUBTYPE {
	TYPE_A,
	TYPE_B
    }
}
 
interface State<T extends Task> {
    public Transition<T> getTransition(JETEvent<?> event);
}
 
class DefaultState<T extends Task> implements State<T> {
    public Transition<T> getTransition(JETEvent<?> event) {
    	return null;
    }
}
 
class FSDState extends DefaultState<FSDTask> {
	@Override
	public FSDTransition getTransition(JETEvent<?> event) {
		if(event == null || ! (event instanceof FSDEvent)) { // <-- doesn't compile
			return null;
		}
		FSDEvent fsdEvent = (FSDEvent)event; // <-- doesn't compile either
		return null;
        }
}
 
interface Task<T extends Task> {}
 
class FSDTask extends DefaultTask<FSDTask> {}
 
class DefaultTask<T extends DefaultTask> implements Task<T> {}
 
interface Transition<T extends Task> {}
 
class DefaultTransition<T extends Task> implements Transition<T> {}
 
class FSDTransition extends DefaultTransition<FSDTask> {}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Casting to Object first solves the problem. The two problematic lines must be

if(event == null || ! ((Object)event instanceof FSDEvent)) { // <-- This compiles
FSDEvent fsdEvent = (FSDEvent)(Object)event; // <-- This now also compiles

for the compilation to work

Release Regression From : 5.0u11
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.

Release Regression From : 5.0u11
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.