Microsoft/TypeScript

Nested destructuring does not narrow dependent parameters

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#48,378 opened on Mar 22, 2022

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BugDomain: check: Control FlowHelp Wanted

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Description

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

⏯ Playground Link

Playground link with relevant code

💻 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.

Contributor guide

Research direction
Investigate the narrowing logic for destructured variables in control flow analysis, specifically looking at how nested destructuring affects type narrowing when using dependent parameters. The bug is that when destructuring a tuple inside an outer array destructuring, narrowing one variable does not narrow the other. Trace the code path in the checker/ binder to see where the narrowing stops working for nested destructuring patterns.
Tech stack
typescript
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitTypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55