Type predicate can sometimes result in different type than type discrimination

[aslilac]

Bug Report

🔎 Search Terms

predicate, discrimination

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 4.5.2. Test in 4.0 and nightly as well.

⏯ Playground Link

Playground link

💻 Code

function test(x: unknown) {
	if (typeof x === "string" || x === 0) {
		x
		if (x === "hello" || x === 0) {
			x
		}
	}

	if (check1(x)) {
		x
		if (check2(x)) {
			x
		}
	}
}

function check1(x: unknown): x is (string | 0) {
	return typeof x === "string" || x === 0;
}

function check2(x: unknown): x is ("hello" | 0) {
	return x === "hello" || x === 0;
}

🙁 Actual behavior

In the first set of nested scopes, typeof x is narrowed to string | 0 by the first if, and then "hello" | 0 by the second if. This behavior is correct.
In the second set of nested scopes, typeof x is narrowed to string | 0 by the first if, but is narrowed to 0 by the second if.

🙂 Expected behavior

"hello" is a valid part of the subset of string, and should be included in the union type. The types derived from these two methods should be identical.

[MartinJohns]

Perhaps related? #31156

[RyanCavanaugh]

Behavior dates back to at least 3.3... I'm surprised this hasn't been noticed before (I suppose a union UDTG is weirdish).