`noUncheckedIndexedAccess` behaves differently b/w type-level and value-level access

[juhort]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

The output of accessing a property from an index signature is different between type-level access and value-level access.

// @noUncheckedIndexedAccess: true

type IndexSig = {[x: string]: number}
type AccessFromIndexSig = IndexSig[string]
//   ^? type AccessFromIndexSig = number

declare const indexSig: IndexSig;
declare const accessor: string;
const accessFromIndexSig = indexSig[accessor];
//    ^? const accessFromIndexSig: number | undefined

Playground

I expect it to have the extra undefined for the type-level access as well. Is this intentional?

[MartinJohns]

Duplicate of #57794. See also #42810. Literally a 10 seconds search.

[RyanCavanaugh]

This is intentional; see "Design Point: What's the type of T[number] / T[string]" in #39560