This question evolved from discussion in the comments from another Q/A: "Can you use Shape Water on Tidal Wave?".
I've become unsure about how spells with a duration of "instantaneous" work. Specifically, do their effects also happen instantaneously?
An example where it matters:
If one readies the spell wall of stone and has the trigger be "when I see a fireball hurtling towards me I will block it with the spell", when do they actually cast the wall of stone spell?
We know that readied actions occur after their triggers, but there are (at least) two possibilities.
The effects of fireball happen instantaneously, so you could cast wall of stone only after every part of the spell, including the fireball exploding and dealing damage.
The effects of fireball happen in a timed order, so you could cast wall of stone immediately after any part of the spell, including the fireball coming towards you.
Can a readied action be triggered and taken after a spell is cast, but before it deals damage?
Or rather, which (if any) of the two interpretations above applies to spells with the duration "instantaneous"?