nested destructured tuple value is string | undefined when second tuple item is not accessed

[G-Rath]

Bug Report

When using noUncheckedIndexedAccess, the type inferred by my code is affected by whether I access the second item in a tuple or not.

🔎 Search Terms

tuple, undefined, type, omit, noUncheckedIndexedAccess, error, index, array, infer, nested

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about noUncheckedIndexedAccess
• I was unable to test this on prior versions because --noUncheckedIndexedAccess didn't exist before those versions

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const fn: <T extends string[]>(t: T) => [T, string[]];

const [[x]] = fn(['1']);

console.log(x.toLowerCase());

It seems that the inferred type for fn is:

const fn: <string[]>(t: string[]) => [string[], string[]]

Since the first item is an array rather than a tuple, items in that array have type string | undefined.

If I change the assignment to be:

const [[x], []] = fn(['1']);

It gets inferred as:

const fn: <[string]>(t: [string]) => [[string], string[]]

Since the first item is a tuple rather than an array, items in that tuple have type string.

🙁 Actual behavior

x has type string | undefined

🙂 Expected behavior

x has type string.

[G-Rath]

It seems like I can resolve this by changing the extends type to be string[] | [string] without any changes in or loss of functionality.

It's very possible that this is The Correct Type I should be using for typing this the most accurately, but will leave this issue open for now as it still strikes me as a little odd that accessing the second item changes the inferred type of the first item, given it's redundant in JS (which is what got me here: eslint rules telling me it's not needed).

[RyanCavanaugh]

That doesn't look right to me either. Contextual inference candidates from destructurings are weird, though, so it's hard to say.

[Andarist]

It seems like I can resolve this by changing the extends type to be string[] | [string] without any changes in or loss of functionality.

This works because it hints the compiler to infer a tuple for T. So if you are aiming to infer a tuple there then this makes sense.

To hint at inferring a tuple I would use this though:

declare const fn: <T extends string[]>(t: [...T]) => [T, string[]];

Somewhat surprisingly - this one infers ["1"] whereas the one that you proposed infers [string].

It's definitely weird to me that adding a pattern for the second item in this destructuring makes it infer a tuple. But then... I'm not sure what the bug is. Should the tuple be inferred for const [[x]] = or should the tuple not be inferred for const [[x], []] =? 😅