Constrained generic class property has any type after instanceof

[OliverJAsh]

🔎 Search Terms

• Type parameters
• Class
• instanceof
• any

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAECyCeBRATsg9sgPAFWgUwA8AXPAOwBMYIjkBLUgcwD5oBvAKGmgAcBXAIxC1g0IgC5o2ANydowNKWrJewIhgAU4yQEo2srkQAWtCADoi0ALyiZXAL7sH7cnlBhkeOQur5UGCQgo6FhK9MzQAD7QvKQA1qRoAO6kMuy0AGbQ6nh+yND01GCkwHhomYG5uhxcOcHmttAA9I1cXAB6APyyzfgE3K4k5BKhjN0tYKq8YCASRfDQgDLkjkA

💻 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.

[jcalz]

Your MyClass<string> | unknown confused me for a minute into thinking you were asking for something else (i.e., that MyClass<X> | unknown should be narrowed to MyClass<X> if you use instanceof, but that's not possible because unknown absorbs everything). Maybe you can just change it to unknown, 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 in T, that the constraint should be used instead of any when narrowing via instanceof? And if it were contravariant you'd expect never, and if it were invariant or otherwise problematic then any would be fine?

[MartinJohns]

Duplicate of #17473.