Can Catholic priests disclose the fact that someone has seen him for confession?

Let me first point out that I think the the answer by AthanasiusOfAlex is correct by the letter. I would like to give a broader interpretation by Bishop Barron in one of his videos on this topic.

He speaks about Can. 983 §1 which he cites from memory as

The canon law of the church stipulates that one may never consciously violate the seal of confession. It's an absolute law within the church's structure. (1:56)

He the continues to explain this law by an anecdote:

Maybe some Non-Catholics, maybe Catholics themselves, but certainly Non-Catholics, might not grasp why this is so important to us, why confession has this very sacred quality.

Let me put it in context by telling a story from my own seminary formation: We took a course in our fourth year at the seminary in confession. And our teacher said this to us and it's been kind of burned in my mind all the years I've been a priest.

He said, if someone asks you: "Father, can you hear my confession?" The answer is always: "Yes!" If it's inconvenient, the answer is always: "Yes!" Even, if it puts your life in danger, the answer is always: "Yes!"

Then he added, and if someone ever says to you: "Hey, what was said to you in that confession?" The answer is always: "It never happened." If saying or acting as though if it never happened is inconvenient: "It never happened." If acting as though it never happened puts you in danger of your life, the attitude is still: "It never happened."

That's how sacred we consider the so called "seal of confession". (3:42)

The bishop then proceeds to explain why this is so important in the context of the sacramental nature of confession:

We hold that confession is not so much an encounter with the priest, it's an encounter with Christ through the sacramental mediation of the priest. In the confessional, a penitent approaches the Lord, and confesses sin to Christ, and receives---through the sacramental mediation of the priest---the healing and forgiving grace of Christ.

Because that encounter is so sacred, we don't want anything to stand in the way of it: If someone would feel "oh-oh, if I say something in the confessional, the priest is likely to repeat it", he or she will not approach that fond of grace. We don't want anything to stand in the way of ones access to the source of salvation. Which is why we take the seal of confession, the privacy of confession with such extraordinary seriousness. (5:04)

So to answer your question: The letter of the law does not directly forbid to disclose the fact that someone was seeing him, the priest, for confession. But Bishop Barron advises the priest to act as if this confession had never happened - even under difficult circumstances. This he sees as an important part of not violating the seal of confession.

To give a personal interpretation: As 3961 notes in the comments, this might be understood to require the priest to lie when asked if someone confessed to him. This might seem ad odds with the eighth commandment, but the Church teaches that the severity of the sin has to be judged with the sinners intention in mind. See for example the wikipedia site (Ten Commandments in Catholic theology) on the eighth commandment for the differences the intention could make.

However, [the eighth commandment] does not require a person to reveal a truth to someone who does not have a right to know, and teaches respect for a right to privacy. (Wikipedia: Ten Commandments in Catholic theology)

An this is (in my opinion) exactly the "attitude" the bishop talks about: Act as if it didn't happen. If someone asks, try not to lie to him. If that person persists and puts the priest in a situation where he either has to lie or would break the seal of confession---then do under no circumstances break that seal. That is because, in this situation the priest is obliged to protect the higher good: By breaking the seal of confession he would do great harm in (prospectively) hindering people to the forgiving grace of God. By lying, he only refuses to answer a question that should not have been asked in the first place.

Church law does not explicitly prohibit revealing that a given person has been to confession or not. Here are the relevant canons from the Code of Canon Law:

Can. 983 §1. The sacramental seal is inviolable; therefore it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner and for any reason.

§2. The interpreter, if there is one, and all others who in any way have knowledge of sins from confession are also obliged to observe secrecy [though not technically the seal; i.e., they would not fall under the latae sententiae excommunication of Can. 1388].

Can. 984 §1. A confessor is prohibited completely from using knowledge acquired from confession to the detriment of the penitent even when any danger of revelation is excluded.

§2. A person who has been placed in authority cannot use in any manner for external governance the knowledge about sins which he has received in confession at any time.

Here, some observations should be made: the confessor may never break the seal of confession. To “break the seal” means to make a third party understand that a given penitent has committed a certain sin.

Hence, while revealing that someone has been to confession generally does not violate the seal, common sense says that it is best not to provide information that could inadvertently reveal a person’s sins—and revealing whether a person has been to confession or not could potentially do this.

(For example, suppose that there is someone whom a lot of people suspect has committed a grave sin—say, adultery or something like that. If I reveal that he has gone to confession, many people will likely conclude, rightly or wrongly, that this person has, in fact, committed the sin. That is the sort of thing that needs to be avoided.)

If I were asked point blank whether someone has gone to confession or not, I would simply reply the way witnesses are trained to do: “I don’t recall.”