Type narrowing regression with switch + for in TypeScript 4.9

[mtreder]

Bug Report

It is possible to generate a "Variable 'X' is used before being assigned." error on TypeScript 4.9 where it previously did not in TypeScript 4.8. This seems to be when we have combination of switch statements and for loop.

🔎 Search Terms

switch
for
type check

🕗 Version & Regression Information

• This changed between versions 4.8.4 and 4.9.3
• This continues to error with nightly.

⏯ Playground Link

TypeScript 4.8.4 Playground Example

TypeScript 4.9.3 Playground Example

Nightly Playground Example

💻 Code

function functionA(): "A" | "B" {
	return "A";
}

function functionB(key: string): string {
	return key;
}

export function functionC(): void {
	let unionVal = functionA();
	switch (unionVal) {
		case "A": {
			break;
		}
		default: {
			return;
		}
	}

	for (let line = 1; line <= 10; line++) {
		let key: string;
		switch (unionVal) {
			case "A": {
				key = "AA";
				break;
			}
		}
		functionB(key);
	}
}

🙁 Actual behavior

Variable 'key' is used before being assigned.

🙂 Expected behavior

No error

[jcalz]

bisected to #51095

[ahejlsberg]

Simpler repro:

declare function functionB(key: string): string;

function functionC(): void {
	let unionVal: "A" | "B" = "A";
	while (true) {
		let key: string;
		switch (unionVal) {
			case "A": {
				key = "AA";
				break;
			}
		}
		functionB(key);
	}
}