assertThrows(
        IllegalStateException.class,
        () -> {
          FluentFuture<Closeable> unused = closingFuture.finishToFuture();
        });
  }

  public void testFinishToFuture_preventsFurtherDerivation() {
    ClosingFuture<String> closingFuture = ClosingFuture.from(immediateFuture("value1"));
    FluentFuture<String> unused = closingFuture.finishToFuture();