Fake nudes of real people are generally illegal, regardless of if the nude is real, or photoshopped, or AI generated.

People have been arrested and convicted for AI porn of real people.

For now convictions seem to be confined to people who have already created/used more traditional CSAM (hidden cameras). This could just be because it is hard to catch someone simply generating images, so if someone with no record would be jailed for just fake nudes remains an open question. Fake nudes of fictional people are also very much an open question. Being very new technology, new laws have yet to be made, so feel free to write to lawmakers about where the line should be.

The short answer is that we don't know. It has long been a legal grey area when depicting porn of underage characters that are clearly not real. There's the classic "she's actually 1000 she just looks 6 years old" and "all characters depicted are at least 18 [despite all of them being in various stages of high school]". American politicans are by and large way out of date with modern technology and cannot be expected to rule competently on the subject. Is the AI nude of a real person "revenge porn" or constitutionally protected free speech? People have been drawing fan art of underage celebs having sex with high detail for a long time (see Harry Potter/Twilight rule 34). Ultimately congress will need to make laws about this or courts will have to interpret a previous ruling as applying to these images. Unfortunately due to the Streisand Effect whomever is at the unfortunate center of this shitstorm will be forever made into pornographic material so we understand the general hesitation to get involved

The following will be a massive oversimplification of the complex laws and court cases over the 20th century trying to grapple with what, quite frankly, is a fairly modern issue. Not the AI aspect, but that of CSAM and how it intersects with American civil liberties (ie the First Amendment).

In the USA, the freedom of speech is very broad, save for very specific, already-established exceptions. These include "imminent threats/fighting words", obscenity (not the same as the dictionary definition), defamation (false statements that tarnish someone's character), and the cause or result of crimes. Whole courses could be taught on just the exceptions to the First Amendment and their contours.

Actual CSAM is exempt from freedom of speech because -- among other reasons articulated by courts -- it can only be produced through abuse of a child, which is a crime. Simulated CSAM, however, has to meet the obscenity standard in order to be exempt, which the Supreme Court articulated as:

The basic guidelines for the trier of fact must be: (a) whether the average person, applying contemporary community standards, would find that the work, taken as a whole, appeals to the prurient interest, (b) whether the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law; and (c) whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

Every word of those guidelines has been deeply analyzed for the 50 years of its existence, and until a better set of guidelines are issued, that's the best guidestar we have. Which is to say, if a lawyer can craft an argument within those parameters, the scenario you've described could indeed be recognized as a crime.

But a small caution: please be very careful when asking to carve exceptions into free speech. As a civil right, it's something which must be jealously guarded, by citizens, lawmakers, and courts. These things are complex precisely because they're trying to avoid criminalizing thoughts and ideas, while also enabling a society to function.

Very short answer with little nuance:

Cops can arrest you for anything and are not required to know the laws. A court will sort that out. If there isn’t a specific law, the court can convict based on similar laws.