throw new AssertionError();
    }
  }

  /**
   * A singleton {@link WeakValueReference} used to denote an unset value in an entry with weak
   * values.
   */
  static final WeakValueReference<Object, Object, DummyInternalEntry> UNSET_WEAK_VALUE_REFERENCE =
      new WeakValueReference<Object, Object, DummyInternalEntry>() {
        @CheckForNull
        @Override