set(uncheckedCastNullableTToT(sourceResult));
      return;
    }

    if (!isInstanceOfThrowableClass(throwable, localExceptionType)) {
      setFuture(localInputFuture);
      // TODO(cpovirk): Test that fallback is not run in this case.
      return;
    }

    @SuppressWarnings("unchecked") // verified safe by isInstanceOfThrowableClass
    X castThrowable = (X) throwable;
    T fallbackResult;
    try {