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

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.