Why these generics don't compile in OpenJDK7, but do in OpenJDK6

class HasId<I> {}
class HasStringId extends HasId<String> {}
class Alert<T extends /*Some*/Object> extends HasStringId {}
class BaseController<M extends HasId<String>> {
    // abstract Class<M> getModelClass();
}
class AlertController extends BaseController<Alert> { // error here
    // @Override Class<Alert> getModelClass() {
    //     return Alert.class;
    // }
}

compiles fine on OpenJDK6, but in OpenJDK7 gives:

AlertController.java:50: error: type argument Alert is not within bounds of
    type-variable T
class AlertController extends BaseController<Alert> {
                                        ^
  where T is a type-variable:
    T extends HasId<String> declared in class BaseController

Note that there's rawtype warning at line 50, because Alert must be parameterized. If I do that, e.g. extends BaseController<Alert<Object>>, code compiles. But I cannot do that, because I need to implement getModelClass().

UPDATE: That was a bug in Java 6 implementations, which was fixed in Java 7: https://bugs.java.com/bugdatabase/view_bug?bug_id=6559182. (And here's my question to compiler devs: http://openjdk.5641.n7.nabble.com/Nested-generics-don-t-compile-in-1-7-0-15-but-do-in-1-6-0-27-td121820.html )