java.util.concurrent Flow is an implementation of reactive-streams specification and as such must conform to rule #9 for Subscription:

"While the Subscription is not cancelled, Subscription.request(long n) MUST signal onError with a java.lang.IllegalArgumentException if the argument is <= 0. The cause message MUST include a reference to this rule and/or quote the full rule.

The intent of this rule is to prevent faulty implementations to proceed operation without any exceptions being raised. Requesting a negative or 0 number of elements, since requests are additive, most likely to be the result of an erroneous calculation on the behalf of the Subscriber."

However, current javadoc states that:

    /**
     * Adds the given number {@code n} of items to the current
     * unfulfilled demand for this subscription. If {@code n} is
     * negative, the Subscriber will receive an {@code onError}
     * signal with an {@link IllegalArgumentException} argument.
     * Otherwise, the Subscriber will receive up to {@code n}
     * additional {@code onNext} invocations (or fewer if
     * terminated).
     *
     * @param n the increment of demand; a value of {@code
     * Long.MAX_VALUE} may be considered as effectively unbounded
     */
    public void request(long n);

i.e. only negative values of n are treated as erroneous.
--------------------------------------------------------------------------------
This issues was created as a result of this discussion: http://cs.oswego.edu/pipermail/concurrency-interest/2017-March/015642.html