Another question to bother Donald with (sorry). This is something that some of us have mused about before, but it never mattered until now. Champion, Merchant and Sauna say "when you play x". But when do you do it, before or after you resolve the played card?
The one situation I can think of is: Play Sauna; then play Silver, trashing a Catacombs, gaining a Black Market/Storyteller, playing it via Innovation. Now it matters if the Silver has been resolved or not. (Edit: Also the number of Actions you get from playing Champion, which matters for Diadem.)
We have some groups of...things:
1) Reactions like Moat, plus Urchin - "when you play x, first"
2) Champion, Merchant and Sauna - "when you play x"
3) Adventures tokens - "when you play x, first"
(Excluded here are cards that say "when you play this" (Noble Brigand, Philosopher's Stone, etc.) since that's part of resolving the play ability, not a triggered ability.)
Reactions/Urchin used to say just "when you play x", but still happened before the played card. Then Adventures came along with tokens that said "first", and later 2nd editions of Reactions/Urchin also said "first" to clarify the timing. Presumably Champion, Merchant and Sauna never got this wording because it didn't matter.
Coin of the Realm, Royal Carriage and Citadel indicate that they happen after the played card. But Champion, Merchant and Sauna don't say either way.
So currently we have some things that say "first" and others that don't. This is the same situation as after Adventures came out, and of course Reactions/Urchin nevertheless happened before the played card. Going by this, it would seem that Sauna should too.
On the other hand, the intuitive interpretation for most players would probably be that the Silver produces $2 before you trash a card.