Forgive me if this has been asked elsewhere - if it has, my google-fu has failed me.
CR 601. Casting Spells handles the process of moving a card from the player's hand, a hidden zone, to the stack, a public zone. Specifically, the process is:
- 601.2a-d: propose the casting of the spell by moving it to the stack and announcing modes, targets, etc.
- 601.2e: game checks the legality of the proposition, reversing if the spell is illegal to cast.
- 602.f-h: Costs are paid. This includes an opportunity for the player to activate mana abilities to pay associated mana costs.
Case 1 - Mistimed casting of Emry, Lurker of the Loch
Consider the case of a player attempting to cast Emry, Lurker of the Loch in a mistimed manner. Player has 2 colorless lands and a Chromatic Star. What the player wants to do is propose the casting of the spell, let 601.2a-d check the legality of the spell, let 601.2f apply cost reduction, then crack the Star to cover the colored mana of the spell.
What the player actually does is crack the Star for {U}, then propose the casting of Emry. The spell cannot legally be cast because the play is unable to produce enough mana to cast the spell, and the game reverts to the state immediately before the proposal.
Case 2 - Cast Down on an illegal target
Consider too the (simpler) case of a player attempting to cast Cast Down on a Smuggler's Copter) that the player did not realized was uncrewed and thus not a creature.
My instinct is that because step 1 in this whole process is to move the card to the stack, the card is revealed to all players regardless of whether or not it is actually legal to cast. Is that correct?
(I'm aware that there are probably tournament rules prohibiting a player from doing this intentionally - players are responsible for maintaining the game state after all. I'm interested in how the rules engine outlined in CR handles this)