Incorrect type narrowing for tuples

[Florian-Mt]

🔎 Search Terms

"narrow tuple", "tuple inference"

🕗 Version & Regression Information

This is the behavior in every version I tried in the playground (v3.3, v4.5, v5.8, nightly), and I reviewed the FAQ about "tuple", "narrow", "inference".

⏯ Playground Link

https://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgCrIN4F8BQOED2IAzmMmADbCkBcyA2ugD7IgCuFFANGsi+5wC6yALwMB3VhwqCcAejnIA8gGlkwGOSql1xZAApmUzgEp6wgEZsyIAmRgEo5NgAcKEYjytkAksgC2EHAgyHDObigWEAhwbMQo7iAA5mAAFrrIMMAAHhAAJngwbCAIYMBEyKnBee76lNRgdIw8qIImmDjIXVoNOLg4Ggb1pAB0EABu0ACedaIAfOTIAIQiYhIm7Rid3VUgNRB12mAmfXiDhw30AAzCK2vSyABkjz2k9ACMt6vGFJvbXbt9hdSCdcEA

💻 Code

interface T {}

const tlist: [T | null, T | null] = [null, null]
// OK if tlist is (T | null)[] but not for tuples, but I mean a tuple because length is fixed

function handle(tlist: [T, T]) {
    tlist
}

if (tlist.every(t => t !== null)) {
    handle(tlist)
}

if (tlist[0] !== null && tlist[1] !== null) {
    handle(tlist)
}

🙁 Actual behavior

handle(tlist) throws in the TS compiler:

Argument of type '[T | null, T | null] & T[]' is not assignable to parameter of type '[T, T]'.
  Types of property '0' are incompatible.
    Type 'T | null' is not assignable to type 'T'.
      Type 'null' is not assignable to type 'T'.

🙂 Expected behavior

The condition in both if blocks should narrow tlist to the type [T, T], i.e. no error.

Additional information about the issue

It is OK if tlist is (T | null)[], but not for tuples. However, I really want to use a tuple here because the length of tlist is known and will never change.

[jcalz]

Your second example is effectively a duplicate of #42384.

---

Your first example is a limitation of the type predicates on array methods like every(). You could merge your own call signature to handle tuples also, maybe like

interface Array<T> {
    every<S extends T>(predicate: (value: T, index: number, array: T[]) => value is S, thisArg?: any
    ): this is { [I in keyof this]: this[I] & S };
}

Playground link

but I'm sure there are weird edge cases. Curiously I searched but I didn't find any existing issues filed about the type predicate methods on tuples, so maybe this isn't a duplicate of an existing issue? There are near-misses about map() on tuples not producing tuples.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.