format() on a discriminated-union error should return a discriminated union

[tavoyne]

Let's say we have two forms on a page. Although they're unrelated, they're processed by the same API point, which uses a z.discriminatedUnion to differentiate them. There's currently no way to know, when there's a parsing error, which one is to blame.

const schema = z.discriminatedUnion("_formId", [
  z.object({
    _formId: z.literal(1),
    firstName: z.string(),
  }),
  z.object({
    _formId: z.literal(2),
    firstName: z.string(),
  }),
]);

const parsed = schema.safeParse(x);

if (!parsed.success) {
  const formatted = parsed.error.format();

  formatted.firstName?._errors; // <-- Is it coming from form 1 or 2?
}

I believe the format() method in this case should return a discriminated union—with the discriminant being the original schema discriminant.

We'd be able to do that:

const parsed = schema.safeParse(x);

if (!parsed.success) {
  const formatted = parsed.error.format();

  if (formatted._formId === 1) {
    formatted.firstName?._errors; // <-- We know for sure it's coming from form 1.
  }

  if (formatted._formId === 2) {
    formatted.firstName?._errors; // <-- We know for sure it's coming from form 2.
  }
}

[akomm]

Now its treeifyError, but same issue:
#5086