Type inference of discrimitated union uses "widest" option

[matthieusieben]

🔍 Search Terms

inference discriminated union narrowing ternary

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

type Unknown = {$type: string}

type B = {$type: 'b'; str: string}
const b: B = {$type: 'b', str: 'foo'}

type A = {$type: 'a'; num: number; embed: B | Unknown}
function isA<V extends {$type?: string}>(v: V): v is V & A {
  return v.$type === 'a'
}

const a: A = {$type: 'a', num: 42, embed: b}

const maybeA: A | B = a

// typed as `Unknown` instead of `Unknown | B`
const result = isA(maybeA) ? maybeA.embed : b

// return type is not `Unknown | B`
function getEmbed(v: A | B) {
  if (isA(v)) return v.embed
  return v
}

Playground

📃 Motivating Example

Better preserve inferred types when working with discriminated unions.

💻 Use Cases

1. What do you want to use this for? A distributed network that allows for custom entity types (atproto)
2. What shortcomings exist with current approaches? Working with data of partially known types is hard(er)
3. What workarounds are you using in the meantime? Explicit typings

[Andarist]

To quote @MartinJohns:

This is working as intended and happens because of type reduction. See #50171 and many many others.

Also: https://www.typescriptlang.org/docs/handbook/type-inference.html#best-common-type

[matthieusieben]

I wasn't aware of the "Best common type" type inference strategy. Thanks for pointing it out. Sorry about the noise 🙏.