Groovy's class verification doesn't allow for covariant return types

import java.util.List;

public interface Base {
    List foo();
}

import java.util.ArrayList;

public interface Child extends Base {
    ArrayList foo();
}

import java.util.ArrayList;

public class JavaChildImpl implements Child {
    public ArrayList foo() {
        return null;
    }
}

class GroovyChildImpl implements Child {
    public ArrayList foo() {
        return null
    }
}