undefined is narrowed away incorrectly

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#60,106 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
typescript
Domain
compilers

Research direction

Start with the linked TypeScript Playground repro and compare its behavior between versions 5.2 and 5.3. Trace the control-flow narrowing for pattern?.IsA() === false; done means the later pattern expression remains A | undefined rather than being narrowed to A.

Written by the indexing model from the issue text.

Description

Bug Domain: check: Control Flow Help Wanted
🔎 Search Terms

narrowing undefined, incorrect type narrowing, incorrect if statement narrowing

🕗 Version & Regression Information
  • This changed between versions 5.2 and 5.3
⏯ Playground Link

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEkoGdnwILwN4CgCQAksugBQCUAXPAC4AWAlmoxgNw4C+OOoSciA9gDtk1eAAco1aiBiCqmAD7wAroNAAzeoJDA261WGr0h8KGLEQAngAVJ02eWz566+CQlSZggPwA6IqRk8AC8ofDqUBDIIEG4eHge9oJs8QD0qfAAet74eHDUyrIpnAl2XinpWTnsQA

💻 Code
declare class A {
	IsA(): this is A;
}

declare const pattern: A | undefined;
function applyPattern() {
	if (pattern?.IsA() === false) {
		pattern;
		// ^?
		return;
	}
	pattern;
	// ^?
}
🙁 Actual behavior

pattern is narrowed down to A, removing the undefined

🙂 Expected behavior

pattern should only be narrowed to A | undefined

Additional information about the issue

No response

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