Historical Rules · 4 February 2011
Rule 304.5
Official source text from the 4 February 2011 Rules release
If text states that a player may do something “any time he or she could cast an instant,” it means only that the player must have priority. The player doesn’t need to have an instant he or she could actually cast. Effects that would prevent that player from casting a spell or casting an instant don’t affect the player’s capability to perform that action (unless the action is actually casting a spell or casting an instant).