Inspired by this question, I'm wondering what happens if want to cast a spell that requires me to see the target, I can see my target in a mirror, and have a clear path to them, but I don't have line of sight to them (for instance, the target is obscured from me by an illusion of a wall, but my helpful mirror-on-a-stick lets me look around the wall.)
The basic rules state:
To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
There's no issue there. The caster has a clear path, no question. But as far as I can see, nothing actually explains how line of sight works. But how do mirrors work in 5e? Do they actually let you see the creature, or just a reflection? The Medusa's statblock says
If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.
In this case at least, the game assumes that the medusa is "seeing itself reflected" instead of "seeing a reflection of itself." This question suggests that seeing a Medusa in a mirror is seeing the Medusa for the purposes of its eyes. So, does this mean I can look around a corner with a mirror and cast, say, Frostbite on an enemy, as long as the wall obstructing my vision doesn't also obstruct my movement?