static <V extends @Nullable Object> V getDoneValue(Object obj) throws ExecutionException {
    // While this seems like it might be too branch-y, simple benchmarking proves it to be
    // unmeasurable (comparing done AbstractFutures with immediateFuture)
    if (obj instanceof Cancellation) {
      Cancellation cancellation = (Cancellation) obj;
      Throwable cause = cancellation.cause;