Type guard on unknown provides incorrect or "stale" editor hints

[alii]

Bug Report

When using type guard on unknown to narrow down a type, we can safely make assertions about an object, but the type information does not carry down for editor hints.

🔎 Search Terms

type, guard, editor, hint, incorrect, inlay

🕗 Version & Regression Information

• Version 4.8.0-dev.20220512
• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

function hasProp<Prop extends string | number | symbol>(
	value: unknown,
	prop: Prop
): value is Record<Prop, unknown> {
	if (typeof value !== 'object') {
		return false;
	}

	if (!value) {
		return false;
	}

	return prop in value;
}

declare const test: unknown;

if (hasProp(test, "foo") && hasProp(test.foo, "bar") && typeof test.foo.bar === "boolean") {
    test.foo.bar;
    // ^? — this is typed as Record<"foo", unknown> when we actually know more info than it is telling us
}

🙁 Actual behavior

When hovering over test or using the // ^? syntax, it's visible that test is of type Record<'foo', unknown>, when really we have already asserted that there is more type information there available to us (e.g. .foo.bar` which is a boolean)

🙂 Expected behavior

The correct shape would appear, e.g.

const test: {foo: {bar: boolean}}

[fatcerberus]

All three of test, test.foo and test.foo.bar are narrowed correctly. The fact that test doesn't become { foo: { bar: boolean } } is expected - in general once you've arrived at Record<K, unknown> (whatever K happens to be), you can't infer anything further about its value type just by checking its individual properties. To put it another way, Record<K, string | number> doesn't suddenly become Record<K, string> just because you can't find any numbers in it right now.

Also, type guards on object properties never narrow the parent object in TS except in the case of discriminated unions. See #31755.

[alii]

Ahh right, that makes perfect sense, thank you very much!

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