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

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.

Beginne mit dem verlinkten TypeScript Playground und reproduziere die beiden Destrukturierungsformen unter noUncheckedIndexedAccess. Verfolge die unterschiedlichen inferierten Typen, die im Bericht angezeigt werden; abgeschlossen ist die Aufgabe, wenn die erste Form x den Typ string gibt und dabei das erwartete Verhalten für die zweite Form erhalten bleibt.