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in operator on T & object narrows to neverย #50639
Description
Bug Report
๐ Search Terms
- in operator
- generic
- intersection
- object
- never
๐ Version & Regression Information
- This changed between versions 4.7 and 4.8
โฏ Playground Link
Playground link with relevant code
๐ป 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`
}
};๐ Actual behavior
See above.
๐ Expected behavior
See above.
Activity
erheron commentedon Sep 6, 2022
It's quite interesting though, since if you remove the
"prop" in valueconditionif,valueis inferred asA & objectas expected ๐คOliverJAsh commentedon Sep 6, 2022
Note that even in TS 4.7 using the
inoperator onA & objectreturnednever: https://www.typescriptlang.org/play?ts=4.7.4#code/MYewdgzgLgBAZgJhgXhgHgIIBoB8AKANwEMAbAVwFMAuGDGAMhhACMArC4KAShRxgG8AUDBEwAlnBh5ho2QCIADgCcQCueLAxi5CjJE8hs2dsoBuPUYD0lmAD0A-BYC+gp6aA. This only became an issue after TS 4.8 started narrowingAtoA & object.fatcerberus commentedon Sep 6, 2022
Since this involves narrowing via
in, this might be related to #50531.andrewbranch commentedon Sep 6, 2022
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
unknownand{}, while for 4.9 we plan to fix the other examples you show here. Anders Hejlsberg (@ahejlsberg) is actively working on this. Iโm not sure thereโs an existing issue tracking it though, so I guess this is it now.