Given `Array<T>` const index which not overflow, should indicate element type `T` instead of `T|undefined`

[loynoir]

Bug Report

🔎 Search Terms

• Array<T>
• const integer index
• not overflow index
• indicate element type from array and array index
• array element type might be undefined

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

function fn(s: string) {
 console.log(s)
}

function reproduce(input: string[]) {
 switch (reproduce.length) {
 case 0 as const:
 case 1 as const:
 fn(input[0 as const])
 break
 default:
 break
 }
}

const input: string[] = []
switch (input.length) {
 case 0 as const:
 case 1 as const:
 fn(input[0 as const])
 break
 default:
 break
}

🙁 Actual behavior

// fn(input[0 as const])
Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
 Type 'undefined' is not assignable to type 'string'.

🙂 Expected behavior

// fn(input[0 as const])
OK

Additional

I guess there is already an issue.

But seems not easy to find which to track on from tons of issues.

[MartinJohns]

Duplicate of Related #38000 (misunderstood what you wanted). Used search terms: array length in:title

Given the example I really don't see any purpose in this. It's easier and cleaner to just access the element and check for undefined.

[loynoir]

Background

Martin Johns (@MartinJohns)

The purpose is parsing column base text into interface.

The column base text definition is in linux kernel.

The column is generated from str.split.

Some columns are required, some columns are optional, based on columns length.

Given

• columns is Array<string> or string[]
• casting columns into interface
• given const-not-overflow-index

Actual

• columns[index] is T|undefined

Expected

• columns[index] is T

Workaround

• columns is LengthCaseArray<string>

// prettier-ignore
type LengthCaseArray<T> =
|[] & {length:0}
|[T] & {length:1}
|[T,T] & {length:2}
|[T,T,T] & {length:3}
|[T,T,T,T] & {length:4}
|[T,T,T,T,T] & {length:5}
|[T,T,T,T,T,T] & {length:6}
|[T,T,T,T,T,T,T] & {length:7}
|[T,T,T,T,T,T,T,T] & {length:8}
|[T,T,T,T,T,T,T,T,T] & {length:9}
|[T,T,T,T,T,T,T,T,T,T] & {length:10}
|[T,T,T,T,T,T,T,T,T,T,T] & {length:11}
|[T,T,T,T,T,T,T,T,T,T,T,T] & {length:12}
|[T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:13}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:14}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:15}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:16}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:17}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:18}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:19}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:20}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:21}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:22}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:23}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:24}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:25}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:26}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:27}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:28}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:29}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:30}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T] & {length:31}
|[T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,...T[]] & {length:number}

[RyanCavanaugh]

This was a well-defined limitation in #39560. We're offering two options here:

• noUncheckedIndexedAccess off, which is convenient in all cases
• noUncheckedIndexedAccess on, which is sound in all cases

Given that we can't fully track all possible array mutations, we don't intend to add a third in-between option, nor unsound behavior in the latter option.

[typescript-bot]

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