in operator on T & object narrows to never

[OliverJAsh]

Bug Report

🔎 Search Terms

• in operator
• generic
• intersection
• object
• never

🕗 Version & Regression Information

• This changed between versions 4.7 and 4.8

⏯ Playground Link

Playground link with relevant code

💻 Code

const f3 = <A,>(value: A) => {
    if (
        typeof value === "object" &&
        value !== null &&
        "prop" in value
    ) {
        value;
        // ^?
        // TS 4.7: `A`
        // TS 4.8: `never`
        // Expected: `A & object`
    }
};

🙁 Actual behavior

See above.

🙂 Expected behavior

See above.

---

[erheron]

It's quite interesting though, since if you remove the "prop" in value condition if, value is inferred as A & object as expected 🤔

[OliverJAsh]

Note that even in TS 4.7 using the in operator on A & object returned never: https://www.typescriptlang.org/play?ts=4.7.4#code/MYewdgzgLgBAZgJhgXhgHgIIBoB8AKANwEMAbAVwFMAuGDGAMhhACMArC4KAShRxgG8AUDBEwAlnBh5ho2QCIADgCcQCueLAxi5CjJE8hs2dsoBuPUYD0lmAD0A-BYC+gp6aA. This only became an issue after TS 4.8 started narrowing A to A & object.

[fatcerberus]

Since this involves narrowing via in, this might be related to #50531.

[andrewbranch]

Yes, related to #50531, but not going to be fixed for 4.8.3. 4.8.3 will contain a more minimal fix specific to unknown and {}, while for 4.9 we plan to fix the other examples you show here. @ahejlsberg is actively working on this. I’m not sure there’s an existing issue tracking it though, so I guess this is it now.