undefined is narrowed away incorrectly

[TrippTrapp84]

🔎 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

[jcalz]

presumably the impl of #55613 let this slip through?

[Andarist]

It's caused by #53714 . I'm investigating this since it's my PR

[Andarist]

I think there were some pre-existing inconsistencies in how optional chains might be narrowed down and how negation impacts them. I continue to investigate this issue and the other ones I found here: TS playground 1, TS playground 2