Incorrect error for unions with literals

Description I use zod@2.0.0-beta.30 I want to use zod to parse structures like

type T = { val: 'a', a?: string } | { val: 'b', b?: number }

but I get incorrect error when I tries to parse structure with incorrect field’s type

Demo

import * as z from 'zod'

const Schema = z.union([
  z.object({
    val: z.literal('a'),
    a: z.ostring()
  }),
  z.object({
    val: z.literal('b'),
    b: z.onumber()
  })
])

console.log(Schema.parse({
  val: 'a',
  a: 123         
}))

Expected error

Error: [
  {
    "code": "invalid_type",
    "expected": "string",
    "received": "number",
    "path": [
      "a"
    ],
    "message": "Expected string, received number"
  }
]

Actual error

Error: [
  {
    "code": "invalid_literal_value",
    "expected": "b",
    "path": [
      "val"
    ],
    "message": "Input must be \"b\""
  }
]

[hmaurer]

I have been using Zod 3 and poor error messages when using discriminated unions lead me to this issue. As @colinhacks pointed out above Zod isn’t able to guess which schema is most “relevant”, but has it been considered to add an explicit “discriminated union” construct to Zod? JSON Type Definition has a discriminator form for specifically this purpose, e.g.

{
  discriminator: "val",
  mappings: {
    "a": {
      properties: {
        a: {type: "string"}
      }
    },
    "b": {
      properties: {
        b: {type: "number"}
      }
    }
  }
}

There’s an AJV implementation as well. In Zod, it could look something like:

z.discriminatedUnion("val", {
  a: { a: z.string() },
  b: { b: z.number() },
});

If there are no immediate objections I’d be happy to work on an implementation.

[alexxander]

I implemented a function for handling discriminated unions (using zod 3.11.6). zodDiscriminatedUnion(discriminator: string, types: ZodObject[])

• discriminator the name of the discriminator property
• types an array of object schemas Its behaviour is very similar to that of the z.union() function. However, it only allows a union of objects, all of which need to share a discriminator property. This property must have a different value for each object in the union.

More info here: https://github.com/colinhacks/zod/issues/792#issuecomment-1023251415