}

  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V, E extends InternalEntry<K, V, E>>
      WeakValueReference<K, V, E> unsetWeakValueReference() {
    return (WeakValueReference<K, V, E>) UNSET_WEAK_VALUE_REFERENCE;
  }

  /** Concrete implementation of {@link InternalEntry} for strong keys and strong values. */