this function argument type can not be narrowed to its subclass

[bogdan]

TypeScript Version: 3.5.3

Search Terms:

Code

type Attributes<T> = {[P in keyof T]? : T[P]};
class A {
  setAttributes(attributes: Attributes<this>) {
  }
}

class B extends A{
  async run(attributes: Attributes<B>) {
    this.setAttributes(attributes)
  }
}

Expected behavior:

No Error

Actual behavior:

tmp/test.ts:9:24 2345: Argument of type 'Attributes<B>' is not assignable to parameter of type 'Attributes<this>'.

I didn't find any way for this check to be relevant. If someone can point me out the use case where such check is required, please let me know.

Playground Link:

http://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=24&pc=1#code/C4TwDgpgBAgswCcCWAjArsCBnAPAFQD4oBeKAbwG0AFKJAOygGsIQB7AMyjwF0B+KAFxdq3AL4BuAFABjADYBDLFljlJUKFgjA4iVBmwAKefGTpMWITtP7cwABZIsBAJSr1oyR5kKlUAEJQEAAemHQAJsowZGpQiiB00lAIaHRGJnrmlulm2Dh+Lm7qUPaOAHSa2tk2abo5WM4xHqJAA

[RyanCavanaugh]

This is a correct error; in a derived class of B, there may be additional properties that would not be present in Attributes<B>

[bogdan]

Extra properties for derived class C from B are not getting accepted by C#run because it is the same as B#run which only accepts Attributes<B>, so it would not be possible to pass them into run. As all properties in Attributes<T> are optional, Attributes<B> will always be a subset of Attributes<this> in the base class.

See line 19:
http://www.typescriptlang.org/play/?ssl=15&ssc=1&pln=17&pc=1#code/C4TwDgpgBAgswCcCWAjArsCBnAPAFQD4oBeKAbwG0AFKJAOygGsIQB7AMyjwF0B+KAFxdq3AL4BuAFABjADYBDLFljlJUKPKFZE9AOYkoAckNT1WCMDg70mLAAp58ZDexCrzjNhzAAFkiwEAJSq6qKSYTIKSlAAQlAQAB6YdAAmyjBkalAoWjp0+qTGphpYIHTSUAhodA5OqJ5YbnUuuDFBIepQvv4AdOaWzQ211g2BWWERcorKAMLxSRCpynGZ6tK5yPkGReGSknQQAO5QM3aBPVU1ZJpG8oYANNlChigPUOtG0oaigUA

[RyanCavanaugh]

Again, this is a correct error. Extra properties is just one form of subtyping that could occur here.

type Attributes<T> = {[P in keyof T]? : T[P]};
class A {
  a: string = '';
  setAttributes(attributes: Attributes<this>) {
  }
}

class B extends A{
  b: string = '';
  async run(attributes: Attributes<B>) {
    this.setAttributes(attributes)
  }
}

class C extends B {
  b: "foo" = "foo";

  setAttributes(attributes: Attributes<this>) {
    if (attributes.b !== undefined) {
      // Must be true, but is false
      console.log(attributes.b === "foo");
    }
  }
}

const b: B = new C();
b.run({ a: 'a', b: 'not foo' });

[bogdan]

Wow! I see what you mean now, thanks for explanation.