When you play Priest and then trash a card without putting the card in the trash, what happens?
Example 1: Play Priest, and then play Chapel trashing Fortress. Do you have +$2 or +$4?
Example 2: Play Priest, and then play Captain to play Acting Troupe from the Supply, failing to trash it. Do you have +$2 now or +$4?
Example 3: Play Priest, and then play Throne Room to play Pixie twice choosing to self-trash Pixie both times. Do you have +$4 or +$6? (Assuming your boons do not grant money or do any trashing)
As Scolapasta said, in example 1 you
did, in fact, trash Fortress. Fortress doesn't say "this can't be trashed", it says "when you trash this, put it into your hand"
The key point is that "trash" means "move the card to the trash". Examples 2 and 3 are not trashing, because in example 2, the card stays in the Supply, never going to the trash, and in example 3, it's
already in the trash on the second play - it can't go
to the trash because it's already there - Fortress on the other hand,
does technically go into the trash, it just immediately leaves it