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Regression: Rest parameter typed as union of tuple types behaves differently in 4.3 #44093
Description
Bug Report
🔎 Search Terms
tuples rest parameter regression
🕗 Version & Regression Information
- This changed between versions 4.2 and 4.3-beta
⏯ Playground Link
Playground link with relevant code
💻 Code
class EventEmitter<ET> {
// works with overloads
// off<K extends keyof ET>(...args: [K, unknown]): void
// off<K extends keyof ET>(...args: [K | unknown]): void
off<K extends keyof ET>(...args: [K, unknown] | [K | unknown]):void {}
}
function once<ET, T extends EventEmitter<ET>>(
emittingObject: T,
eventName: keyof ET,
callback: (event: unknown) => void,
): void {
emittingObject.off(eventName, callback); /* fails*/
// type assertion works as well
emittingObject.off(eventName as keyof ET, callback);
}🙁 Actual behavior
The call emittingObject.off(eventName, callback); fails with Argument of type '[string | number | symbol, (event: unknown) => void]' is not assignable to parameter of type '[unknown] | [keyof ET, unknown]'.
🙂 Expected behavior
The call should succeed as it did in 4.2
Activity
RyanCavanaugh commentedon May 14, 2021
Trimming off the last little bit of fat:
It looks like something's gone wrong here where we construct a contextual type for argument position
0(correct) and then decide to use the presence of that to devolveeventNamefromkeyof ETto its constraintstring | number | symbol(bad). But it's something specific to the tuple syntax, since equivalentish forms likeoff<K extends keyof ET>(args: K | boolean, a2: string | number):void {}don't trigger this.ahejlsberg commentedon May 17, 2021
This is caused by #43183. The issue is that the contextual type of the first argument to
offisK | unknown, which ends up reducing to justunknown. Sinceunknowncontains no generic types we think it is safe to reducekeyof ETto its constraintstring | number | symbolwhich would be a candidate for narrowing by CFA. The fix is to ensureK | unknowndoesn't get reduced. We do that elsewhere for contextual types, just not in the particular scenario of arguments to rest parameters with union types.dragomirtitian commentedon Jun 2, 2021
Daniel Rosenwasser (@DanielRosenwasser) Ryan Cavanaugh (@RyanCavanaugh) This does not appear fixed in 4.3.2 (Playground Link) but works in nightly (Playground Link). Will there be a 4.3.3 where this will be included ?