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Constrained generic class property has any type after instanceof #60810
Description
🔎 Search Terms
- Type parameters
- Class
instanceofany
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics.
⏯ Playground Link
💻 Code
class MyError<T extends string> {
public t: T;
constructor(t: T) {
this.t = t;
}
}
declare const error: MyError<string> | unknown;
if (error instanceof MyError) {
error.t;
// ^?
// expected: string
// actual: any ❌
}🙁 Actual behavior
See above.
🙂 Expected behavior
See above.
Additional information about the issue
Potentially related?: #17253. Although, this one seems to be for anonymous classes, which is not the case here.
Activity
jcalz commentedon Dec 18, 2024
Your
MyClass<string> | unknownconfused me for a minute into thinking you were asking for something else (i.e., thatMyClass<X> | unknownshould be narrowed toMyClass<X>if you useinstanceof, but that's not possible becauseunknownabsorbs everything). Maybe you can just change it tounknown, since that's the same thing and there's no implication of narrowing to a union member?Is the idea that since
MyClass<T>is covariant inT, that the constraint should be used instead ofanywhen narrowing viainstanceof? And if it were contravariant you'd expectnever, and if it were invariant or otherwise problematic thenanywould be fine?MartinJohns commentedon Dec 18, 2024
Duplicate of #17473.