Bug: Predicate function does not narrow correctly

[mozesstumpf]

🔎 Search Terms

predicate

🕗 Version & Regression Information

It worked till version version 5.4.5

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.3#code/JYOwLgpgTgZghgYwgAgILIN4FgBQBIMATwAcIAuZAIjkoG5cBfXXUSWRFAIU1wJPKoAjOo2Y5W0eEmQBhHviKkKlBCJxMcLcJI7IAIvL5KqAEzUbcilKm4BeNMgA+yTvU04rsg-bnO9by340ABtg5HsbJy8AnAQAexAAZzBkOEEKSPsAWTgwAAsAOig4EBM4gFsACgBKZAA+ZAAGAoBWZAB+TGQrZRpkBmQKDG7+ZWF+mPiklLhQjNDw1MFJhOTkYESZb2RK2eD54NrbBux8KAgwAFcoEFTQgs9bJ6pVJ2c9h6Cn+0ozNwYYsAYDsNltdqFqrVTng9rRkAB6eFoBBXWYUEAQABu0AANMgAKIAD1IKIgJgoW1EOCAA

💻 Code

interface A {
	type: "a";
}

interface B {
	type: "b";
}

interface C {
	type: "c";
}

interface D {
	type: "d";
}

type AB = A | B;

type CD = C | D;

type All = AB | CD;

const ab: AB = Math.random() > 0.5 ? { type: "a" } : { type: "b" };

const all: All = ab;

const isCD = (all: All) => {
	return all.type === "c" || all.type === "d";
};

if (isCD(all)) {
	all; // Actual: never, Expected: CD
}

🙁 Actual behavior

all's type is never inside the if statement

🙂 Expected behavior

all's type should be CD inside the if statement

Additional information about the issue

When we initialize the all, we explicitly give it the type All (even tho we assign an AB to it), and when we hover on it, it shows that its type is All.
But, when we check it with the predicate function (isCD), it looks like that it tries to compare the AB type instead of the All, and its result will be never since AB and CD has no overlap.

[MartinJohns]

This is working as intended. Assignment narrows, the compiler can see that all can possibly only be A or B, so after checking both with your predicate the resulting type is never.

You can avoid the narrowing upon assignment using a type assertion: const all: All = ab as All;

[mozesstumpf]

Yep, your right. The error on my code is actually caused because the assignment itself that does not narrowed correctly inside the while loop, but that's another bug, so it can be closed