[v4] Zod v4 infers branded keys into unbranded keys

Open jthemphill opened this issue 6 months ago • 4 comments

Reproduction

In packages/zod/src/v4/classic/tests/record.test.ts, try adding this test case:

test("branded keys type inference", () => {
  const recordWithBrandedNumberKeys = z.record(z.number().brand("SomeBrand"), z.number());
  type recordWithBrandedNumberKeys = z.infer<typeof recordWithBrandedStringKeys>;

  expectTypeOf<recordWithBrandedNumberKeys>().toEqualTypeOf<Record<number & z.$brand<"SomeBrand">, number>>();
});

Expected Behavior

We think Zod should infer this type:

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

Actual Behavior

But if you ask TypeScript what we infer fromrecordWithBrandedStringKeys, you'll see that we actually infer this type:

type recordWithBrandedNumberKeys = {
    [x: string]: number;
}

The key has morphed from a branded number into an unbranded string! This is a regression from how Zod v3 behaved.

Screenshots

Viewing the inferred type

Hovering over `recordWithBrandedNumberKeys` to see the type Zod infers

Viewing the test failure

Hovering over the result of `expectTypeOf<>` to show the error we get when we expect the key to be branded

May 21 '25 17:05 jthemphill

Probably related to this now in the docs?

May 21 '25 19:05 Svish

Nice find, but this is not related to that entry in the docs! If you remove .brand() from the key, then Zod has no problem creating a Record<number, number>.

As the docs say in your screenshot, "Zod allows it for the sake of parity with TypeScript's type system."

Zod4 is not properly handling branded keys - a branded anything falls through to just being a string, and this is not how the unbranded types are treated.

May 21 '25 20:05 jthemphill

This is a bug in your test: you're inferring the type from recordWithBrandedStringKeys instead of recordWithBrandedNumberKeys.