This question stems from the following question about casting a spell through Scrying, and specifically about a comment made on an answer I shared.
Can you cast a spell with a range of "Sight" on a target you can see, even if you don't have a clear path to the target? In other words, as long as you can see the target, even if there is an obstruction in the way, can you still cast a spell with a range of "Sight" on them?
No I wouldn't say this is possible. In the PHB it states
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.
so you can AOE the side of the obstruction (i.e. target the side that you have clear path to) but you need a clear path to directly target.
A WotC podcast also clears targeting up: You always need a clear path to target a creature with a spell. You don't necessarily need to be able to see them (i.e. heavy fog wont impede) but the travel path of the spell must be clear. The spell originates from the caster so the travel path from the caster to the target must have a clear travel path. Full cover stops travel path (including glass).
