Is it legal to cast a non-cantrip 1st-level spell like Cure Wounds with an action, and then use the sorcerer's Quickened Spell Metamagic option to cast a cantrip like Eldritch Blast as a bonus action?
I was told by a DM that I was unable to do this as the rules contradict it "By RAW". I have looked over the rules and do not see where this is stated.
The sorcerer's Quickened Spell Metamagic option allows you to cast an action spell as a bonus action instead:
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
But if you cast any spell as a bonus action, the only other spell you cast that turn has to be a cantrip:
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.
However, cure wounds is not a cantrip, so you can't cast it as an action on the same turn as you cast a bonus-action spell.
If you use Quickened Spell on cure wounds (to cast it as a bonus action) and cast eldritch blast as an action, you'd be fine, ironically. Because eldritch blast is a cantrip.
The Bonus Action Spellcasting rules, found in the Player's Handbook on page 202, specify that if any spell (cantrip or non-cantrip) is cast as a Bonus Action during a character's turn, then the only spells they are allowed to cast for the rest of that turn are Cantrips that have a casting time of 1 action. So their ruling was correct.
However, Eldritch Blast is a cantrip which requires (normally) an action to cast. If you were to flip it around (Quicken the Cure Wounds and then regularly cast the Eldritch Blast), it would be 100% legal to cast both spells by rules as written. Moreover, there's no requirement in the rules that a Quickened spell must be cast after a regular spell (or vise-versa) so you could still cast these spells in the order you tried to cast them. You just have be more careful about how you apply the metamagic.
The confusion you ran into with this rule is confusion that I've run into many times as DM, to the point where I have explicitly Houseruled, at my table, that this rule no longer applies in any 5e games I run. At my table, characters (both PCs and NPCs) are allowed to cast any two spells as Bonus Action and Action, without restriction on the levels of the spells being cast—though, obviously, you still have to legally be able to cast a spell as a Bonus Action, either because that's its normal casting time (like Misty Step) or because you have access to Metamagic/other features that allow Bonus Action spellcasting.
There's three main reasons I made this change. The first is that the D&D 5e designers are on record as saying that the reason they wrote this rule in the first place was to avoid Decision Paralysis at the table—but I have personally found that we actually get the exact opposite problem: players end up getting confused and taking long times to make their turns because the rule is awkwardly written and a little unintuitive, and are constantly asking "okay, I want to cast like this, is this allowed?", especially where Sorcerers are concerned. The Decision Paralysis problem got much better (it happened less often) after we nixed the rule.
The second reason I chose to get rid of this rule is because removing the rule offers a very substantial power boost to Sorcerers, a class that I think is somewhat undercooked in the 5e rules. And I can confirm that at my table, even with substantial Homebrew to boost their Sorcerer features and this rule removed on a character that did take the Quicken Spell metamagic, the sorcerer ultimately ended being pretty average on the scale of power within the party. So the fact that removing the rule boosts Sorcerers [who choose to take the Quicken Spell Metamagic] substantially is not something I see as a problem.
And the final reason is that there's a very unintuitive interaction with this rule, which is that you also lose the ability to cast spells as Reactions during the turn. It doesn't come up often, but you do end up with Counterspell Wars (i.e. an NPC tries to Counterspell the PC's spell, and they try to Counterspell in return), and if this rule is in place, then the PC is unable to use Counterspell if they've cast a Bonus Action spell. Because, again, the rule states that the only other spells that may be cast are Cantrips with a casting time of 1 action. This situation can be a little marginal, but when it does come up, the interaction with this rule has always been frustrating to my players.
So for those reasons, I would suggest you float the idea of nixing the Bonus Action Spellcasting rule to your DM. I find it to be cumbersome to adjudicate at my table, find it really doesn't offer value for its intended purpose, and I feel the way that its removal affects game balance is acceptable.
However, you cannot quicken the cantrip, and then cast cure wounds normally.
The goal of the rule is to avoid someone casting Fireball twice in a single turn (or some bonus action spell + regular spell), but allow someone to quicken a spell and then take the Dash action, or quicken a spell and then cast a Cantrip, or cast a bonus action spell and also cast a cantrip in the same turn, etc.
The situation where both spell and cantrip are normally regular Actions, but you are using a class ability to make one into a Bonus Action, is especially confusing as it means to do both you must make the spell into a bonus action and leave the cantrip alone.
It would not break any balance considerations to simply houserule any combination of spell + cantrip to be allowed in a single turn, regardless of which is the Bonus Action and which is the Action.
Relevant rules text;
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.
As a side note, that your GM stuck to the letter of the rules instead of informing you of this interaction indicates they may not understand these rules, or may think that cantrips cannot be cast on the same turn as other spells (despite the exception specifically written into the rules for this). Draw his attention to the relevant text if necessary, and if need be explain that the rules consider cantrips to be more like melee or ranged attacks than the more powerful 1st level and higher spells.
RAW you can't do this, but you can do it the other way around: quicken the leveled spell, cast the cantrip as your action.
It's often seen as silly that action-non-cantrip + bonus-cantrip isn't allowed, while the reverse is. But it has at least two consequences. I'd guess the first of these is why the rule exists.
You can't use most other metamagic on the spell you Quicken. e.g. Careful Fireball + Quickened Fire Bolt doesn't work, but Quickened Fireball + Twinned Fire Bolt is allowed.
If you want to Quicken a spell and use another metamagic option on the same turn, the other one has to be applied to the cantrip. (If one spell was already a bonus action like spiritual weapon, you wouldn't need Quickened Spell and could apply other metamagic to it as well as a cantrip. Same if you use action surge to cast two 1-action spells.)
It's not technically allowed to cast a leveled spell as an action first, and then see the result before deciding to quicken a cantrip as a followup, spending or saving 2 sorc points.
Many DMs would be fine with this in practice, even if they know the rules details. Or let you retroactively quicken the first spell, if you didn't use any metamagic on it. But some DMs might choose not to, especially if a SorLock is outshining other PCs with Quickened Eldritch Blasts.
If you started with an action Cantrip, you can follow up with a quickened leveled spell, fully RAW. But a 1-action leveled spell is incompatible with casting anything as a bonus action, including a quickened cantrip. And casting anything as a bonus action (cantrip or not) locks you out of casting any more spells that turn except for 1-action cantrips. Either of those things commits you to that choice for the turn.
If neither of these "exploits" apply or you aren't worried about them, you might simplify the rule to only one leveled spell per turn (except via Action Surge, although RAW that still doesn't allow you to cast action + action + bonus unless both actions are cantrips.)
RAW, though, casting a bonus-action spell locks you out of casting a reaction spell that turn. (e.g. Shield from an op attack, Counter Spell a Shield or Counterspell, Silvery Barbs, etc.) This is often seen as an unnecessary or unintentional side-effect of how the rules wording is designed. But counterspelling a counterspell on your turn can be quite powerful, perhaps it makes sense to make that exclusive with casting a bonus-action spell. RAW is relatively clear, so it's mainly an issue of whether you want to keep that restriction if trying to simplify the bonus-action spell rules for ease of play. (Like "one leveled spell per turn" is easy for players to remember and apply.) Reaction spells later the same round are not locked out, only on your actual turn.
Other Q&As:
An answer on the canonical Q&A Which uses of Bonus-Action spellcasting are legal? has the full RAW details including Action Surge and Reactions. (Most of the other answers, and the table in the question, omit reactions.)
