Does a rogue attacking from behind a corner as a creature passes by have advantage?

Question

Reading about the use of the Hide action in the PHB, and reading answers (1, 2, 3) on the topic had me wondering: if a rogue hiding behind a corner attacks a creature passing by with a melee weapon, would the first attack occur quickly enough for the rogue to be considered as unseen, and therefore as having advantage? I'm considering that in this scenario, the rogue remains motionless until the creature crosses the corner, arriving at 5 feet of the rogue when the attack is made.

Answer 1

You are confusing when you can hide with when you can be hidden

The hiding rules on page 177 PHB say:

You can’t hide from a creature that can see you, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position

As Dale's answer wonderfully demonstrates, being in sight and being hidden at the same time are possible. What counts for hiding is if the creature can see you while you take the hide action: that is not possible, so you can't actively "hide in plain sight", but you can be hidden in plain sight. While you are behind the corner, the creature cannot see you, so you can hide.

The hiding rules continue:

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.

In your case, there is no need to approach the creature, as it is passing by right next to you. You can directly attack it, and would gain advantage on the attack from being hidden.

Answer 2

You are hidden “[u]ntil you are discovered or you stop hiding”

The rules are clear that there are only two things that end the hidden ‘condition’: being discovered or stopping hiding.

So, you are hidden around the corner and a creature comes around the corner.

So what?

the question isn't whether a creature can see you when you are hiding. The question is whether it can see you clearly.

You are still hidden unless they discover you. You will be discovered by a particular creature when their active or passive as applicable Wisdom (Perception) equals or exceeds your Dexterity (Stealth) check. Not just because you no longer have cover. We’ve all had the experience of being startled to suddenly notice someone who was standing in plain sight.

Please consider the following important inequality:

$$\text{Hidden}\ne \text{Out of Sight}$$

The sniper in this photo is hidden but they are not out of sight:

Wally is hidden in this image but he is not out of sight:

Conversely, the dog shit on the bottom of this shoe is out of sight but it’s not hidden if you can smell it:

Unless their passive perception equals or exceeds your Dexterity (Stealth) roll, you will be unseen when you attack, giving advantage. Note that if there is only dim light, they will have a -5 on their passive perception due to disadvantage unless they rely on senses other than sight.