Hi Guillaume, I wanted to better explain the reason why I think the issue with <default-package> is a bug.
If I express the configuration with java annotations instead of XML configuration then the java compiler won't throw exception that MyConstraint is not found in java.lang.* package which is included by default from the java compiler. So the following will work:

import java.lang.*; //e.g. default package

@MyConstraint //located in default package
public class MyBean {
}

where MyConstraint will be put inside java default package (e.g. no package) - the compiler won't complain that MyConstraint is not inside java.lang package as if it is not found then it will try default java package as well.

So I think that whatever you are allowed to express as java annotations is the same thing that you should be able to do with XML configuration. Hope that's explain why I think this is an issue