Can a Bladesinger wizard cast a Bonus Action spell and still use their Extra Attack feature to cast a cantrip on the same turn?

Question

I noticed a peculiar rule interaction. The rule on casting bonus action spells states:

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.

But, consider that I am a 6th-level Bladesinging wizard, giving me the Extra Attack feature (TCoE, p. 77):

Starting at 6th level, you can attack twice, instead of once, whenever you take the attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.

Once I cast a Bonus Action spell such as misty step, can I still use Extra Attack to cast a cantrip (e.g. fire bolt) on the same turn, even though I've changed its casting time from 1 action to 1 attack?

Answer 1

Yes, for cantrips with a casting time of 1 action

The casting time is an attribute of the spell, not of the action you use to cast the spell. Unless a feature you use explicitly tells you it is changing the casting time of the spell (like for example, the sorcerer's Metamagic option Quickened Spell), it won't change the casting time of the spell.

That means you can cast any cantrip that has a casting time of 1 action with your extra attack action, after you cast any spell as a bonus action.

Answer 2

Yes, for most cantrips

The wording of the bonus action spell rule only requires the cantrip have a casting time of 1 action, and the bladesinger rule doesn't specify the casting time actually changes. Curiously enough, this excludes the few cantrips which don't have a casting time of 1 action - shillelagh, magic stone and mending.