Currently, .optional() behaves like:

const user = z.object({
  username: z.string().optional(),
});
type C = z.infer<typeof C>; // { username?: string | undefined };

This results in a type mismatch when you're using --exactOptionalPropertyTypes in TypeScript 4.4 and expecting type C to match an interface definition like:

interface User {
  username?: string;
}

.partial(), .partialBy(), .deepPartial() all have the same issue.

It would be nice to unbundle the optionality of the key from the union type with undefined for the value.

I suggest that these methods be changed to specify the optional absence of the key by default, and perhaps accept an option to restore the old behavior of adding .or(z.undefined()) to the value schema(s). This would unfortunately be a breaking change, but it makes more sense than the current behavior, especially as more projects adopt --exactOptionalPropertyTypes.