How does casting a spell while casting a spell affect priority?

Question

So, in this question, a process for casting a spell while you're in the middle of casting another spell is described, and the consensus in the answers is that it works, and allows you to cast a second spell while you're in the process of casting a second spell.

What I want to know is how this affects the passing of priority. Normally, when you cast a spell, you complete the process of casting a spell, and it is put on the stack before passing priority to your opponent.

However, in this case, you interrupt this process by casting a second spell, which would normally require you to pass priority to your opponent before the first spell's casting process can resume, right? How would the fact that you have a partially-cast spell affect the priority passing?

Would your opponent be able to affect the first spell with counterspells before you've finished casting it? Does your second spell become uncountable because you can't pass priority until the first spell has finished being cast? Do you finish casting both spells before passing priority with both of them on the stack?

Answer 1

You don't do anything with priority after casting the "inner" spell.

It doesn't really matter that you're casting the spell in the process of casting another spell. There are many different ways that you could end up casting a spell during the resolution of another effect, and in all of those cases, you are not using priority to cast the spell, so you don't pass priority when you finish casting it. This is stated in the last step of casting a spell, rule 601.2i:

Once the steps described in 601.2a–h are completed, effects that modify the characteristics of the spell as it’s cast are applied, then the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, they get priority.

In the linked scenario, the player casting the spell did not have priority before casting it because they were in the middle of resolving the activated ability of Selvala, Explorer Returned.

Answer 2

It doesn't have any effect on priority

There's a little bit of a misunderstanding of how priority moves in the question, even normally you don't need to pass priority between casting spells. The rules for priority moving are in section 117.3

117.3 Which player has priority is determined by the following rules:

117.3a The active player receives priority at the beginning of most steps and phases, after any turn-based actions (such as drawing a card during the draw step; see rule 703) have been dealt with and abilities that trigger at the beginning of that phase or step have been put on the stack. No player receives priority during the untap step. Players usually don’t get priority during the cleanup step (see rule 514.3).

117.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.

117.3c If a player has priority when they cast a spell, activate an ability, or take a special action, that player receives priority afterward.

117.3d If a player has priority and chooses not to take any actions, that player passes. If any mana is in that player’s mana pool, they announce what mana is there. Then the next player in turn order receives priority.

This all boils down to priority moving in three ways:

1. The game moves to a step players gets priority. (117.3a)
2. A spell or ability on the stack resolves. (117.3b)
3. The player with priority decides to do nothing with it. (117.3d)

Casting a spell and adding it to the stack doesn't move priority at all. (117.3c). Panglacial Wurm is odd because it allows you to cast it due to its own ability. This would work no matter what causes you to search, even if it's from something like an opponent's Collective Voyage, which makes all players search their libraries for land. Either way no player has priority at that time, but Wurm does allow itself to be cast. This is because of rules 117.2e and 608.2g:

117.2e Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, “Resolving Spells and Abilities.”

608.2g If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a–i, except no player receives priority after it’s cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.