Can a readied range attack provoke an attack from the intended target?

Question

Pathfinder 1e rules question...

A and B are medium sized creatures. A is a ranged character, B is a melee character.

Let's say A readies a ranged attack at anyone that comes into visibility. No enemies are visible nor within range at the moment.

Let's say B (an enemy of A) appears to A as part of their move + attack turn. B appears by moving adjacent to A -- perhaps because B is in fog/stinking cloud and comes within 5 feet, or perhaps the room is in darkness and A only has a candle illuminating 5 feet.

So A gets to use their readied action -- they see B, and they shoot (with appropriate miss chances and such). Does B get an AoO in addition to their planned attack?

I would guess that no, B does not get an AoO, for at least 1 of 2 reasons:

• B becomes visible to A as B is entering the adjacent square, and thus I believe A's attack would interrupt B's movement and B is technically not in melee range when the ranged attack is made.

• I'm not even sure you can make an attack of opportunity when it is your turn, or, if you can AoO when you are triggering the readied action on your turn. (Can B even take AoOs on their turn in PF1e?)

There is a similar question for 4e supporting my guess, but I'm not familiar with 4e and am unaware of the rules differences. That question is here: If a readied ranged attack action is used against the appearance of a burrowing creature, does the attack provoke an attack of opportunity?

However, I couldn't find a discussion/ruling for PF1e -- either my Google skills failed me or it hasn't been asked.

Answer 1

B can make their opportunity attack

Your two reasons don't check out by the rules. I will assume, as you seem to, that nothing allows B to see more than 5ft away (I think a better example would have been keeping the burrowing example from your linked question) even with concealment.

• B becomes visible to A as B is entering the adjacent square

This one is kind of hard to prove directly, specifically because of the strange wording on Ready and more specifically

The action occurs just before the action that triggers it.

The trick here is that A is not triggering their Attack off of an action. They are triggering based on a state of combat which is that they ready to make an "attack at anyone that comes into visibility".

This trigger is not fulfilled until B is completely adjacent to A. Because of the approximation of combat into 5ft squares, it may seem strange that A cannot attack as B enters the square adjacent, but if you're asking mechanically, there is no rules about concealment that allow your sight to include creatures entering your visible spaces. Essentially, the adjacent square is the fog/shadow that can be seen through and being in those 5ft is them emerging from them, which does not mechanically start at the line; B is either 10ft and unseen away or 5ft away and concealed/visible (depending on the effect). Remember that 5ft (plus the amount of space between the archer and the 'edge of their 5ft' is actually a fairly large amount of space. Because magic, anything beyond that might as well be behind a wall. But also the game can only simulate so much. Fog doesn't mysteriously block sight at exactly 5x ft away from us, 5ft is a good amount of room to be able to 'reach', etc.

Sometimes this is helpful (avoiding AoO's from Huge creatures, for instance) and other times (most situations you want to make ranged attacks) it is a hindrance.

If you want a decent idea of what this looks like (and meet the R rating), try to find and watch the climactic fight from the 2002 movie Gangs of New York.

• I'm not even sure you can make an attack of opportunity when it is your turn

This one's actually super easy. You can absolutely use your Attack(s) of Opportunity on your own turn.

You threaten all squares into which you can make a melee attack, even when it is not your turn.

The corollary (assumption, even) is that you threaten squares on your turn.

An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you.

No restrictions about 'on their turn', 'when its not your turn', or anything else.

An attack of opportunity is a single melee attack, and most characters can only make one per round. You don’t have to make an attack of opportunity if you don’t want to. [...] If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).

Again, no restrictions about whose turn the AoO is made on.