Can a Slowed creature cast a spell that costs a bonus action on its next turn?

Question

The rules on casting a spell with a casting time of 1 bonus action state:

[...] You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

The slow spell description says

If the creature casts a spell with a time of 1 action, roll a d20. On an 11+, the spell can't take effect until the creature's next turn, and it must use its action on that turn to complete the spell. If it can't, the spell is wasted.

A creature fails its save and is now slowed. The creature attempts to cast a spell and I rolled a 11+, so the spell is delayed until its next turn. If next turn it uses its action to complete the spell, can the creature still cast bonus-action spells on that turn?

Arguably, the action taken on its next turn is "complete the spell", not "cast a spell", so the creature can still cast bonus-action spells, hence my question.

Answer 1

Action economy while under slow effects

Slow has a lot of very specific (and awesome) debuffs, but the relevant ones to this are regarding how it treats spellcasting:

On its turn, it can use either an action or a bonus action, not both

Here, the creature under the slow effect has to decide if they're going to take their action or a bonus action.

They can't do both, but at this point, casting a bonus action spell will work with the only penalty that they can't then take an action as well.

Casting an Action cost spell

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.

If the creature rolls an 11+, then they must spend their action on Turn 1 to cast and not have a Bonus Action available. They then must use their action on Turn 2 to complete the spell, also making a Bonus Action unavailable.

**This means that if you cast a single action spell and roll an 11+, then you can not cast a Bonus Action spell (or use another action) until Turn 3.

Bonus Action casting alone

What is most important with the slow effects is that there are no changes to bonus action casting.

You can cast that bonus action spell as a bonus action with the only applied relevant debuff being that you can not then take an action.

Want to guarantee a spell goes off in a single round while under slow effects? Cast it as a bonus action.

Answer 2

Only if you don't complete the first spell (or slow wears off)

NautArch correctly identified that the portion of slow which causes a 1 action spell to be delayed until the following turn, does not cause you to "cast" that spell on the following turn but rather "use your action to complete it". We are in full agreement that this won't fall afoul of the rule regarding bonus action spells on the same turn as action spells. However, he did not at all address how the other effect of slow interacts.

Slow has this effect in addition to the one which delays spell casts:

On its turn, it can use either an action or a bonus action, not both.

The consequence of this is simple: If you use your action to complete the spell then you aren't allowed to take a bonus action at all, let alone cast a spell with it. Alternatively, you could use your bonus action to cast something else (bonus action spells don't have a chance of being delayed) but you would "waste" the spell you cast the previous turn.

If slow wore off between those two turns then you're all set. The language for spell delay doesn't have an exceptions for slow ending so you would still need to take an action to complete it, but at that point you wouldn't have cast a spell so you can additionally cast a spell as your bonus action.