E.g. for the following decorator:

@Decorator
public abstract class InstanceDecorator implements Instance<Foo> {

    @Inject
    @Delegate
    Instance<Foo> instance;

}

the set of decorated types contains Instance<Foo>, Iterator<T>, Producer<T>.

See also TCK test:
org.jboss.cdi.tck.tests.decorators.builtin.instance.BuiltinInstanceDecoratorTest