Validate any combination of z.record() in a union?

[paulrudy]

I'm trying to use a union of records like this:

const schema = z.union([
  z.record(z.number()),
  z.record(z.literal('test'), z.string()),
]);

Objects that satisfy only one member of the union are valid: e.g., const config = { foo: 1, bar: 2 } and const config = { test: 'horse' }; are both valid.

But objects that combine members of the union are invalid. e.g., const config = { foo: 1, test: 'horse' }; is invalid. Why?

Is there another way that I can achieve this combining of record types?

[paulrudy]

After referring to this discussion, I discovered that .catchall() is one solution to the specific example:

const schema = z
  .object({
    test: z.string(),
  })
  .catchall(z.number());

It would still be interesting to understand the answer to my question:

But objects that combine members of the union are invalid. e.g., const config = { foo: 1, test: 'horse' }; is invalid. Why?