Type narrowing for predicates works with implicit if, fails with explicit if

[ljharb]

Bug Report

See https://tsplay.dev/Nr8YVW - it seems like the type narrowing allows truthy/falsy checks, but doesn't support when the predicate's return value is explicitly compared to true or false.

🔎 Search Terms

i guess type narrowing, predicates?

🕗 Version & Regression Information

This occurs in the latest nightly on the TS playground; see above. I assume it's always been the case since predicate support was added, but haven't checked.

⏯ Playground Link

https://tsplay.dev/Nr8YVW

💻 Code

function isFunction(x: unknown): x is Function {
	return typeof x === 'function';
}

const x: unknown = function () {};

if (isFunction(x)) {
	x;
	// x is a function here
}

if (!isFunction(x)) {
	x;
	// x is not a function here (should be more specific than `unknown`, but ¯\_(ツ)_/¯)
}

if (isFunction(x) !== false) {
	x;
	// x should be a function here
}

if (isFunction(x) === true) {
	x;
	// x should be a function here
}

🙁 Actual behavior

See above.

🙂 Expected behavior

See above.

[fatcerberus]

Duplicate of #44366

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.