Bounded wildacard generated into buggy Java code in joint compilation

Given a Groovy Interface like:

public interface TestObject<T>  {
  Class <? extends TestObjectFactory> getFactoryClass()
  void setFactoryClass(Class <? extends TestObjectFactory> clazz)
}

(TestObjectFactory is a simple generic marker Interface: interface TestObjectFactory<T>{})

and a Java class using this interface in any manner, Idea detects no problems.

But when compiling via Ant I get the following compilation error:

TestObject.java:16: > expected
java.lang.Class<TestObjectFactory extends TestObjectFactory> getFactoryClass();
(and many, many more of the same kind)

If I compile the whole thing without the Java class, it seems to work fine - but when I put the resulting jar into another project's classPath and try to extend any (or at least all I tried so far) of the contained Groovy classes with a Java class, Idea expects me to implement invokeMethod(), setMetaClass(), getMetaClass(), setProperty() and getProperty().

rather weird - and definitely a show stopper here, because I'm currently the only one on this team using Groovy and my Java-using colleagues need (i.e.: insist on) correct generics as a means of documentation...