Type narrowing fails to account for multiple conditions

[ljharb]

Bug Report

🔎 Search Terms

type narrowing

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

https://tsplay.dev/mAJ91W

💻 Code

function isNaN(x: unknown): x is number {
	return x !== x;
}
function SameValue(x: unknown, y: unknown): x is typeof y {
	if (x === y) { // 0 === -0, but they are not identical.
		if (x === 0) { return 1 / x === 1 / y; }
		return true;
	}
	return isNaN(x) && isNaN(y);
};

🙁 Actual behavior

The y in 1 / y has a type error.

🙂 Expected behavior

Since this is inside a block where x === y, and also where x === 0, TS should be able to statically know that x and y are of type number (since both of them is definitively a zero, either +0 or -0). I shouldn't need the y === 0 check just to please TS.

[fatcerberus]

function isNaN(x: unknown): x is number

For the record, this is wrong as it implies that x is NOT number if the function returns false (UDTGs are treated as exhaustive). See #15048.

function SameValue(x: unknown, y: unknown): x is typeof y

This simply expands to x is unknown. You'd need to make it generic to do what this signature implies, but even then you'd probably run into #15048 again.

[ljharb]

@fatcerberus thanks, that's fair about the isNaN thing - unfortunately TS doesn't allow is NaN.

the x is typeof y isn't the important part; replace it with boolean and the same type error appears for this issue, but thanks for the pointer :-)

[fatcerberus]

Yeah, I figured the return signature of SameValue was irrelevant to the main issue, but didn't want to develop any misconceptions about how type predicates work, lest another duplicate of #15048 shows up 😉