title: Error setting a default null value for an annotation's field
body: <p>Why am I getting an error "Attribute value must be constant". Isn't <strong>null</strong> constant???</p>
  
  <pre><code>@Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  public @interface SomeInterface {
      Class&lt;? extends Foo&gt; bar() default null;// this doesn't compile
  }
  </code></pre>