Edit: And apparently I've been ninjaed by the man himself. I'll leave this here for posterity (because I think it would be correct given previous rulings, and because it took a long time to type out), but obviously it doesn't gainsay Donald's
new ruling.
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My thought was this:
1. Both effects (drawing a card, and discarding the MS for gold) are both triggered by precisely the same exact event, the trashing of the OE.
The two effects can be triggered at the same time. However, in order for them to be triggered at the same time, they must actually be triggered at the same time--which means the MS has to be in hand at the time of trashing. Consider two cases.
First, this is what happens if MS is in your hand:
You use Chapel to trash the OE. Market Square is in hand, and you reveal and discard it. Two events have been triggered: drawing a card, and gaining a Gold. These effects both happen after the OE is trashed (though the trigger is initiated before. You may resolve those effects in either order. Therefore, if you also had a Watchtower in hand, you could resolve MS first, reveal WT in response, top-deck the Gold, then resolve the effect from OE, drawing the Gold.
This is what happens if MS is not in hand, but on top of your deck:
You use Chapel to trash OE. One event is triggered: draw a card. This happens after the trashing is complete. You draw the MS. The OE has already been trashed, so you cannot reveal and discard the newly-drawn MS; it has no trashing to react to.
In other words, the specific order of events in Case 1:
Hand: Chapel, Overgrown Estate, Watchtower, Market Square, Gold
Play card: Chapel.
Trigger Chapel action: trash OE.
Trigger On-trash event: draw a card.
Reveal and discard MS.
Trigger On-trash event: gain Gold.
All triggers have been pulled.
Trash OE.
Resolve all on-trash effects at the same time.
Choose one effect: Gain Gold.
Reveal Watchtower
Trigger on-gain event: choose to trash or top-deck.
All triggers have been pulled.
Gain Gold.
Choose: Top-deck Gold.
Second on-trash effect occurs: draw a card, which is the top-decked Gold.
All actions have been used.
Trigger: transition to Buy phase.
Begin buy phase. Hand: Watchtower, Gold, Gold.
Buy: Gold
Trigger: Gain Gold
Reveal Watchtower
Trigger On-gain effect: choose to trash or top-deck.
All triggers have been pulled
Gain Gold
Choose: Top-deck Gold
End of turn.
In case 2:
Hand: Chapel, Overgrown Estate, Watchtower, Silver, Gold. Card on top of deck is MS.
Play card: Chapel
Trigger Chapel action: trash OE
Trigger On-trash effect: draw a card
All triggers have been pulled.
Trash OE.
Draw a card (MS).
All actions have been used.
Trigger: transition to Buy phase.
Begin buy phase. Hand: Watchtower, Silver, Gold, Market Square
Buy: Silver
Trigger: Gain Silver
Reveal Watchtower
Trigger on-gain effect: Choose to trash or top-deck
All triggers have been pulled.
Gain Silver
Choose: trash Watchtower
Reveal and discard Market Square
Trigger on-trash effect: gain Gold
All triggers have been pulled
Trash Silver
Gain Gold
Reveal Watchtower
Trigger on-gain effect: choose to trash or top-deck Gold
All triggers have been pulled
Choose: top-deck Gold.
End of turn.