[v4 beta] Properly support branded keys in records

[jthemphill]

This should resolve the longstanding discussion:

• When `z.infer` a `Record` with `Brand` as a key, it becomes a Partial Record. #2069

I'm running into this problem:

const stringRecord = z.record(z.string(), z.number());

// ✅ Record<string, number>. This is correct.
type stringRecord = z.infer<typeof stringRecord>

const brandedStringRecord = z.record(z.string().brand("SomeBrand"), z.number());

// ❌ Partial<Record<string & $brand<"SomeBrand">, number>>.
// But this is wrong - a branded string should be treated the same way as a string.
type brandedStringRecord = z.infer<typeof brandedStringRecord>;

Colin explains the rationale for sometimes returning Partial<Record<K, V>> in #992 (comment). But that logic doesn't apply to brands - this is just an oversight.

---

@AlexErrant noticed the same problem, and submitted a PR into Zod v3, which this PR is based on:

• recordWithBrandedKeys should not be Partial #3918

I'm going with a different approach than that PR takes, though.

Today, the code says that the record is not partial if one of these is true:

1. All strings are valid keys (string extends K)
2. All numbers are valid keys (number extends K)
3. All symbols are valid keys (symbol extends K)

This PR adds three more checks. The record is also not partial if one of these is true:

4. All branded strings are valid keys (string & $brand<never> extends K)
5. All branded numbers are valid keys (number & $brand<never> extends K)
6. All branded symbols are valid keys (symbol & $brand<never> extends K)

When my change didn't work, I found that the problem was because $brand<T> uses T in an index key signature. This meant that brand<never> resolved to {}, which is not really correct from a type theory perspective.

So... I'm also changing the type implementation of $brand in this PR. I think this is an acceptable change to make while Zod v4 is still in beta.

Let me know what you think!

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[colinhacks]

I may be losing my mind, but this appears to already by working with Zod 4. Can you verify this?

[jthemphill]

I may be losing my mind, but this appears to already be working with Zod 4. Can you verify this?

Hi! I agree that your screenshot shows the current behavior. I would define your screenshot as an example of exactly the problem I am trying to solve. I believe the correct type should be

type brandedStringRecord = {
    [x: string & z.core.$brand<"SomeBrand">]: number;
}

As your screenshot shows, Zod 3 and Zod 4 are both making the record partial, and they weaken the value type into number | undefined, because a branded string is not a string, at least type-wise. This is the correct behavior if the key is an enum, for reasons you've explained before. But branded strings are not enums!