Nested destructuring does not narrow dependent parameters

Bug Report

🔎 Search Terms

destructuring destructured dependent parameters nested control flow analysis

🕗 Version & Regression Information

This changed between versions 4.5.5 and 4.6

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💻 Code

declare const Tuple: [true, string] | [false, number];
const [[val1, val2]] = [Tuple];
if (val1) {
	val2 // string | number
}

🙁 Actual behavior

TypeScript does not correctly narrow val2 when narrowing val1 if it's in any type of nested destructuring.

🙂 Expected behavior

I expect it to behave the same as directly destructuring the object.

Is this the same issue?

type T = { path: ['one'], value: string } | { path: ['two'], value: number }

function foo(payload: T) {
  if (payload.path[0] === 'one') {
    payload.value  // string | number, should be just string
  }
}

This issue is referring to the control flow analysis of destructured unions so I don't believe this is related to that issue.

This is fixed by #56306. The nested tuple binding in the original example now preserves the dependency between the discriminant and its paired value: value2 is narrowed to string after if (value1).

The example still reports TS2322 in 5.4.0-dev.20231128 and is clean in 5.4.0-dev.20231129, TypeScript 6.0.3, and the current native compiler.

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Nested destructuring does not narrow dependent parameters #48378

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🙂 Expected behavior

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Nested destructuring does not narrow dependent parameters #48378

Description

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Activity

jtbandes commented on Jun 21, 2022

Fireboltofdeath commented on Jun 22, 2022

RyanCavanaugh commented on Sep 8, 2026

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