Question

If you switch or faint does Future Sight still attack?

Comments

As far as I know, future sight can be prevented only by these 4 things:
It misses.
The target fainted and the player has not yet replaced it.
The target has a type or ability (like dark type or wonder guard) that makes it immune to future sight's type (usually psychic but can be changed by a few things).
The user and target are the same individual Pokemon. (one way to achieve this is targeting the ally and then using ally switch)

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If the opponent faints, Future Sight stops? So it won't hit the next Pokemon that comes in?

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Replacing a fainted Pokemon happens after most (possibly all) end of turn effects, including future sight.

Answer 1

I tested it on Showdown and Future Sight hit even when the user switched out or fainted.
Replay

If the user is not on the field when Future Sight hits, its Ability and held item are not applied.

This implies that Future Sight will still hit even if the user is not on the field.
Source

Comments

I've been misled all my life