We had a D&D 5E session where a Specter besides a character and a wall "moved" into the wall without leaving the reach of the character, but depending on how you consider reach to work with obstacles and when you leave it, would there be an opportunity attack?
The rules on opportunity attacks state:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.
- So we have normal case where someone Hostile moves past the Defender, and gets hit leaving * since moved out of reach, getting opportunity attacked:
---*->H .D. ...
- Then we have case where someone moves in a circle around the defender - no opportunity attack:
---+ .D| H-+
- So what if someone runs by an open door, or an alcove or something. A tunnel opening by someone with reach like a lance? Is there an opportunity attack (it feels like there should be)? Where? What about 10ft reach? Things are getting a bit weird now, but seems like there should be an attack of opportunity at one of [?]:
---???->H ===.=== ..D.. ..... .....
- Ok, so what if some hostile runs a circle around someone, and there is a wall or pillar or something enough to give 100% giver temporarily? Now we have a case where hostile arguably is better off than the No Opportunity Attack circle (cover some of way), yet is arguably worse off than running across doorway (less cover yet it felt right door runner had got opportunity-attacked. Technically reach not needed if thin iron plate of right size, but for sake of ASCII art:
--?->H ..=.. ..D.. ..... .....
- And the case that got us thinking about it all. Specter beside Defender (no reach) and a wall moves into wall. Would there be an opportunity attack? Specter did not leave reach by distance, yet is not reachable. But if there is an opportunity attack, what if specter just "fell" through floor? Falling by someone does not cause an opportunity attack? By now the whole thing just seem broken.
.-|->S .D| ..|
Any suggestions on what RAW and (perhaps) what common sense says about these cases?