Make type narrowing for destructured discriminated unions work for more types #59657

🔍 Search Terms

discriminant union, ref, control flow guard, type narrowing

✅ 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 Ref<T> = { value: T }
type Data =
    | { ready: Ref<true>, payload: string }
    | { ready: Ref<false>, payload: null }

declare const data: Data
const { ready, payload } = data

if (ready.value) {
    payload    // <== currently inferred as "string | null" but should be "string"
}

Treat types like Ref<T> as a discriminant property in a union or find a way to narrow the type of payload

📃 Motivating Example

This is a very common use case in the Vue Pinia state store library, millions of projects use this library and have code like

const store = useDataStore()
const { ready, payload } = storeToRefs(store)

If we can improve this type narrowing behavior, the narrowed payload type can helps developer write safer code than before

// before, Non-null assertion everywhere
if (ready.value) {
    payload.xxxx()     // <=== false alert raised by typescript and developers have to use ?. or ! to avoid it
    payload?.xxxx()    // <=== ?. is unnecessary, generates dead code and brings cognitive confusion
    xxxxx(payload!)
}
xxxxx(payload!)        // <=== copied from the if block and forget to remove the ! mark, cannot receive alert from typescript

// after, everything works fine
if (ready.value) {
    payload.xxxx()
    xxxxx(payload)
}
xxxxx(payload)         // received the null check protection from typescript

💻 Use Cases

More detailed playground link

The use cases is actually shown in the motivating example.

I've dig into the checker.ts for some time and here's my findings

1. getDiscriminantPropertyAccess cannot treat ready as a discriminant property now because it needs to check CheckFlags.Discriminant which implies CheckFlags.HasLiteralType. It's a pretty strict check and as its name describes, Ref<T> has no chance to pass this check.
2. I'm not sure is it possible for relaxing the discriminate requirements but it seems to be a bad idea after some search. Fix discriminant property check #29110 is what I found but it's a really old PR so maybe time changes now
3. If we cannot solve it by using discriminant property narrowing, as a newbie to the typescript project, I just tried to debug the checker and have another idea

interface Ref<T> { value: T }
type ToRefs<T> = { [K in keyof T]: Ref<T[K]> }
function toRefs<T>(o: T): ToRefs<T> {
    return {} as any
}

interface DataPrepared {
    ready: true
    payload: string
}

interface DataNotPrepared {
    ready: false
    payload: null
}

type Data = DataPrepared | DataNotPrepared

declare const data: Data
const { ready, payload } = toRefs(data)

function isDataReady(d: Data): d is DataPrepared {
  return d.ready.value
}

if (isDataReady(data)) {
  ready.value     // <=== inferred as boolean but should be true
  payload.value   // <=== inferred as "string | null" but should be string
}

function assertDataReady(d: Data): asserts d is DataPrepared {}

if (ready.value) {
  assertDataReady(data)
  ready.value     // <=== inferred as true which is expected but it's narrowed by other code path
  payload.value   // <=== inferred as "string | null" but should be string
}

Can we use type predicates or assert function to add more information to payload's flow list? If it's possible, maybe we can do following steps while examine the payload

1. check payload's symbol, if its declaration is a BindingPattern
2. check the flow list for payload, if the narrowed data is the initializer of payload's declaration
3. narrow the payload based on the narrowed data
4. maybe it's gibberish but hope it helps