Class<X> exceptionType,
      final AsyncClosingFunction<? super X, W> fallback,
      Executor executor) {
    checkNotNull(fallback);
    AsyncFunction<X, W> asyncFunction =
        new AsyncFunction<X, W>() {
          @Override
          public ListenableFuture<W> apply(X exception) throws Exception {
            return closeables.applyAsyncClosingFunction(fallback, exception);
          }

   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
   * primary input fails with the given {@code exceptionType}, from the result provided by the
   * {@code fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has
   * failed, so if the primary input succeeds, it is never invoked. If, during the invocation of