If your bonus-action spell is counterspelled, are you still restricted to casting a cantrip that turn?

Question

The rules for bonus-action spells state:

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, counterspell seems to interrupt the casting of a spell thus preventing it from completing and causing it to fail:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.

So, if indeed the spell was never completely cast, are you still restricted to casting a cantrip with a casting time of 1 action that turn?

Answer 1

RAW is unclear

Based on the language use of interrupt and spell fails and has no effect, there can easily be confusion in interpreting the final effect of a cast or uncast spell.

This question covers a lot of the discussion, especially between the two top rated answers, but we do have some clarification from Jeremy Crawford on Twitter.

Crawford Clarifies the order of events

In a tweet that covered the question linked in the paragraph above, Crawford states (emphasis mine):

When successful, counterspell foils a spell that is cast. The casting isn't undone. The spell fails and is wasted.

It is clear Crawford believes a spell that has been counterspelled is still cast.

If cast, then regular spellcasting restrictions persist

If you have used your Bonus Action to cast a non-cantrip spell, then you can only cast a cantrip with your Cast a Spell action.

Answer 2

You are restricted to the cantrip

The PHB has the following rule:

If a specific rule contradicts a general rule, the specific rule wins.

I emphasise this, not because there is a general vs specific conflict, but instead to reenforce how we should interpret the rules.

The general rule for casting a spell (from the PHB) states:

When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively "filling" a slot with the spell.

The general rules don't provide for a difference between attempting to cast a spell, and casting a spell. Thus the salient mechanic of casting a spell is the expenditure of a spell slot.

Counterspell is a specific rule that modifies how a spell is cast. As such we have to assume that the general rule (ie that you have cast the spell) still applies, unless Counterspell specifically modifies it.

The relevant text of Counterspell is (emphasis mine):

You attempt to interrupt a creature in the process of casting a spell.

If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.

...

On a success, the creature's spell fails and has no effect.

The specific rule of Counterspell says that the spell fails and has no effect. Importantly it does not say that the casting of the spell is undone (or some other variant of this).

The plain reading of something failing is that an action is performed but it doesn't succeed. The action is not reversed, it fails to succeed.

It also specifically says that the creature is casting a spell.

Additionally the casting time specifies when you can take the reaction to cast Counterspell:

1 reaction, which you take when you see a creature within 60 feet of you casting a spell

You don't prevent it from casting the spell. In other words you don't preempt the casting, you cause their casting to fail instead, by interrupting some vital portion of the spell.

To put it up another way, Counterspell modifies the result, not the initiating action. In particular it does not give the original caster their spell slot back.

So, you have cast a spell as a bonus action, but the spell failed. Thus the rules surrounding casting a bonus action spell apply. In particular the other spell you cast must be a cantrip.