Function returning never in branch doesn't narrow type if it's not explicitly annotated

[nicolo-ribaudo]

🔎 Search Terms

function callback never type narrowing inference

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwxgE8AFWAZxAAowAjALniqlpxg0aoEp4BeAPnioQANxAweA+GgDWqHAHdUXRiJxZgAbgBQ2jEQAOCAIJ94AbwKGQjAETHb8AL479R+ACEzltzfi2PRxddUEhYBDA8cgwhHFBGUwAfTx09YjIYSmZWdj5Bc214IvgWNgwdYvgAeir4AD0Afl1KrEQmeVAAOl8+Xl5-Bx5S9m5UyrgMZBh8DoRyKGxyRCwQchTq2tsAFWt4AHIkzz34YBxV2Jj5xcQiAgALBBAADyNMEGArdz2PPdttJy4ukIpAo1GyZU4kkEwjEEjyFkKxVa7TiIG6uz6-XstiGOQwo2axQmUxmqPgVywSxWaw8OgB2iAA

💻 Code

declare function tryParse(cb: (abort: () => never) => unknown): void;

type A = { type: "A" };
type B = { type: "B" };

declare const node: A | B;

tryParse(abort => {
    abort;
    // ^?

    if (node.type === "A") abort();

    return node satisfies B; // "Type 'A | B' does not satisfy the expected type 'B'"
})

tryParse((abort: () => never) => {
    if (node.type === "A") abort();

    return node satisfies B;
})

🙁 Actual behavior

In the first case, node is not refined to B but it still has type A | B

🙂 Expected behavior

The two usages of tryParse are exactly the same, abort has exactly the same type in both, so they should both narrow the type of node.

Additional information about the issue

No response

[MartinJohns]

This is working as intended and done for performance reasons. See #51075 and many many others.

[nmain]

While not precisely a duplicate of #51075, this is related to that and a known limitation of never narrowing. In order for narrowing to work, the type of the variable being called needs to be known before the inference pass happens.

[nicolo-ribaudo]

Oh thanks 👍