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?
Best Answer
If you see something in a mirror, you do, in fact, see it
First, I'll just note that total cover does nothing here because there is no total cover (as defined in the scenario). If there were total cover, even from something like a window, you would not be able to target the creature with a spell, but there is not total cover, so you can target them.
Now, looking at a spell that requires you to see your target, we need only ask ourselves what it means to "see" something. The rules of the game are written in natural language and when a word, such as "see", does not have a rules-defined definition, it defaults to the English one. I can assure you that any native speaker of English would say that you can, in fact, see things with a mirror. This is also shown in the definition of the Steel Mirror item:
It explicitly helps you "look around corners", and I see no reason to say that it doesn't then also help you "see around corners". Thus, for all intents and purposes, using a mirror can extend your "field of view" and allow you to see things you otherwise could not have seen.
This is perhaps somewhat similar to obtaining x-ray vision or any other form of enhanced sight; you are able to look at something you previously could not have. I honestly can't think of any argument that you can't see a creature that you, well, see in a mirror because the natural language support is so strong here.