feature request: combining/nesting/extending discriminated unions

In our app we have some data coming from an external service that’s modelled as a combination of 2 discriminated unions, e.g.

[
  { type: "quote", attribution: "..." },
  { type: "textAndMedia", mediaType: "image", image: {...} }
  { type: "textAndMedia", mediaType: "video", video: {...} }
]

But attempting to model it with zod fails with the error:
The discriminator value could not be extracted from all the provided schemas

const t = z.discriminatedUnion("type", [
  z.object({ type: z.literal("one"), foo: z.string() }),
  z.object({ type: z.literal("two"), bar: z.string() }),
  z.discriminatedUnion("secondType", [
    { type: z.literal("three"), secondType: z.literal("a") },
  ]),
]);

An extra problem we’ve ran into is the nested discriminator is defined elsewhere, but unlike objects it doesn’t have a .extend method, and a ZodDiscriminatedUnion isn’t a valid arg for ZodObject.merge

const otherDiscriminator = z.discriminatedUnion("secondType", [
  z.object({ secondType: z.literal("a") }),
  z.object({ secondType: z.literal("b") }),
]);
z.discriminatedUnion("type", [
  z.object({ type: z.literal("one"), foo: z.string() }),
  z.object({ type: z.literal("two"), bar: z.string() }),
  otherDiscriminator.extend({ type: z.literal("three") }),
]);

Instead of extending I considered extracting the discriminator options and mapping over them, but zod can't infer that correctly. Code sample in toggle

const otherDiscriminatorOptions = [
  z.object({ secondType: z.literal("a") }),
  z.object({ secondType: z.literal("b") }),
] as const;

const otherDiscriminator = z.discriminatedUnion("secondType", [
  ...otherDiscriminatorOptions,
]);

z.discriminatedUnion("type", [
  z.object({ type: z.literal("one"), foo: z.string() }),
  z.object({ type: z.literal("two"), bar: z.string() }),

  z.discriminatedUnion("secondType", [
    ...otherDiscriminatorOptions.map((option) => ({
      type: z.literal("three"),
      ...option,
    })),
  ]),
]);

[jasonsilberman]

Wanted to bump as having the ability to use a discriminated union with extend/merge would be really, really nice!

[sdirosa]

@maxArturo nice!

I can confirm that your changes to allow using a ZodDiscriminatedUnion as an element of other discriminated unions fixes the issue. Requiring the discriminated unions to share the same discrimination key seems reasonable to me.

In our specific case all of the unions we are discriminating, and occasionally nesting, have the same exact discrimination key. Also, the key is valued uniquely across all objects.