Type guard is not working with noUncheckedIndexedAccess and destructuring

[PunGy]

Bug Report

When noUncheckedIndexedAccess is on and you try to use a type guard like typeof list[index] !== undefined, it would not work if index was created via destructuring assignment or from the tuple.

🔎 Search Terms

noUncheckedIndexedAccess index destructuring undefined tuple

🕗 Version & Regression Information

• Compiler error
• Tested in 4.1.5, 4.5.5, and 4.9.5.

⏯ Playground Link

Playground link

💻 Code

// index is a number
const index = 1
// indexD is a number
const [indexD] = [1]

const tuple: [number] = [1]
const indexT = tuple[0]

const list: Array<number> = [1, 2, 3]

// Block A
if (typeof list[index] === 'number') {
  // `num` has a `number` type
  const num = list[index]
}
// Block B
if (typeof list[indexD] === 'number') {
  // `num` has a `number | undefined` type
  const num = list[indexD]
}
// Block C
if (typeof list[indexT] === 'number') {
  // `num` has a `number | undefined` type
  const num = list[indexT]
}

🙁 Actual behavior

Type guard does not work with the num in the Block B and C

🙂 Expected behavior

num in the Block B and C should have the same type as the num in the Block A

[MartinJohns]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal.

[fatcerberus]

Workaround: Assign list[index] to a const first then type-guard that.

[PunGy]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal.

I see... But still quite strange that even when you declare something as a constant literal, for example:

const [indexT] = [1] as const // here indexT has a type '1', and still doesn't work
const index: number = 1 // index has a 'number' type, not literal 1

if (typeof list[indexT] === 'number') num = list[indexT] // number | undefined
if (typeof list[index] === 'number') num = list[index] // number

Very confusing behavior

[typescript-bot]

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