undefined is narrowed away incorrectly

🔎 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

Commencez par le repro TypeScript Playground lié et comparez son comportement entre les versions 5.2 et 5.3. Suivez le narrowing du flux de contrôle pour pattern?.IsA() === false ; le travail est terminé lorsque l’expression pattern ultérieure reste A | undefined au lieu d’être réduite à A.