How does Payback work against a move that does not effect the target?

Question

For example (I know that speed would not make this work), if a Ursaring uses Slash and afterwards, a Gastly uses Payback, what would the effect be? Would it hit, and if it does, how much damage would it deal?

Comments

Because the answers both prove good points, could someone test this?

Answer 1

Payback inflicts damage. Its power doubles to 100 if the user moves after the target, if the target switches out, or if the opponent uses an item.

source
It says the target has to move. Damage has nothing to do with this.

Comments

After gen 5 it won’t double in power if the opponent switches out.

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Thank you.

Answer 2

Payback will hit regardless if the user took damage beforehand, but it will have 50 bp instead of 100. Payback will not double in damage if the user did not take damage first, so if a move had no effect and didn't hit, it counts as not taking damage and the move will strike with only 50 bp.

In other words, Ursaring will use Slash but the move will fail. Gastly will use Payback, which does hit, but has lower bp than it would if Gastly had taken damage before it used the move.

Comments

The two answers you got suggest two different things. How do you know which one is accurate?

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I'm pretty sure sumwun's answer is correct because he/she/it had a source proving his/her/its answer. But idk.

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Turns out sumwun's right, which doesn't surprise me because he usually is. Payback's BP will double if it moves last, even if it didn't take damage first. I'd suggest the asker of this question reselect his answer as best.

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When I saw this answer, it was late and it made sense. Sorry for this inconvenience and having more people justify 'sunwun's' answer. I guess I'll have to chose the best answer in the morning, rather than 11:30 at night from now on.