Sum types are not diffed correctly

[exFalso]

Given a class SomeClass that has a field of an interface type(or superclass) A and given two instances of SomeClass, one having B extends A in that field, another having C extends A then these aren't diffed correctly (it returns UNTOUCHED) because the property diffing is based on the field's type rather than the actual instances' types.

example:

import de.danielbechler.diff.ObjectDifferBuilder;

class Asd {

    public interface A {
    }

    public static final class B implements A {
        private final int asd = 0;

        public final int getAsd() {
            return this.asd;
        }
    }

    public static final class C implements A {
        private final String fgh = "";

        public final String getFgh() {
            return this.fgh;
        }
    }

    public static final class SomeClass {
        private final A a;

        public final A getA() {
            return this.a;
        }

        SomeClass(A a) {
            this.a = a;
        }
    }

    public static void main(String[] args) {
        System.out.println(ObjectDifferBuilder.buildDefault().compare(new SomeClass(new B()), new SomeClass(new C())));
    }
}