Create an empty directory and copy into it the following two sources:

Ref.java:

public class Ref<T> {
    final RefQueue<? super T> queue = new RefQueue<>();

    public static void main(String[] args) {
        Ref<Object> r = new Ref<>();
        RefQueue<Object> q = r.queue;
    }
}

RefQueue.java:

public class RefQueue<T> {}

Then execute:

    mkdir out
    javac -d out Ref.java

This compiles fine and produces two class files in out directory:

    Ref.class
    RefQueue.class

Now do the following:

    mkdir out2
    rm RefQueue.java
    javac -cp out -d out2 Ref.java

Which produces compile-time error:

Ref.java:6: error: incompatible types: RefQueue<CAP#1> cannot be converted to RefQueue<Object>
        RefQueue<Object> q = r.queue;
                              ^
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Object super: Object from capture of ? super Object
1 error

reported by Peter Levart at compiler-dev: http://mail.openjdk.java.net/pipermail/compiler-dev/2014-January/008457.html