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.