This question already has an answer here:
I upvoted this comment (of necessity, one made by someone else):
because at the time, it applied usefully to a comment by another user. The other user took a hint and deleted their comment, making this comment obsolete. However, I can't flag it as obsolete (or anything else) because I upvoted it.
Isn't that a bug of sorts? Comments can need flagging when circumstances change, even if you upvoted them before the circumstances changed?
I observe that the How do comment voting and flagging work? FAQ specifically states that you can't flag comments that you've upvoted or upvote comments that you've flagged (indeed, it says so twice), but there is no justification given for that design decision. And the proposal put forth by this question is 'that design decision should be revisited for the scenario where you first upvote and later need to flag a specific comment'. (The reverse direction — flag then upvote — doesn't give me cause for concern at the moment.)