This question already has an answer here:
The spellcasting rules say:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
I assumed the order didn't matter, and an unwritten, but logical deduction, is:
If you cast a spell with an action, you can no longer cast spells or cantrips with a bonus action on that turn.
However, a mate has argued with me, that if you first cast a spell with an Action, then you can cast another with a Bonus Action, since casting as an action does not prevent casting as a bonus, only the other way around.
I think it doesn't make sense, but in fact, the rules have some ambiguity. Does "You can’t cast another spell during the same turn" mean "you can no longer cast another spell", or "you can't have cast and you can no longer cast another spell"?