@Override
        public Object waitForValue() {
          return null;
        }

        @Override
        public void notifyNewValue(Object newValue) {}
      };

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {