Extended Class Static methods don't refer to correct this

[sean256]

Bug Report

🔎 Search Terms

• static class this reference

🕗 Version & Regression Information

I can replicate this bug in all versions provided by the typescript playground, including Nightly:

⏯ Playground Link

Playground link with relevant code

💻 Code

class SomeParentClass {
    private foo: string;
    constructor(foo: string) {
        this.foo = foo;
    }

    getFoo() {
        return this.foo;
    }

    static SomeStaticFactory() {
        return new this('hello world');
    }
}

const someParentInstance = SomeParentClass.SomeStaticFactory();
console.log(someParentInstance.getFoo());

class SomeChildClass extends SomeParentClass {
    someNewMethod() {
        return 'it works';
    }
}

const someChildInstance = SomeChildClass.SomeStaticFactory();
console.log(someChildInstance.someNewMethod()); // Property 'someNewMethod' does not exist on type 'SomeParentClass'.(2339)

🙁 Actual behavior

A subclass which uses a Static method of the parent references the wrong this. In the above example, someChildInstance which is returned from SomeChildClass.SomeStaticFactory(); DOES have someNewMethod, but TypeScript barfs on it.

🙂 Expected behavior

I expect TypeScript to understand that the return type from SomeChildClass.SomeStaticFactory(); is an instance of SomeChildClass and not SomeParentClass

[MartinJohns]

Sounds like a duplicate of #5863. You can't type the return value of SomeStaticFactory as this because it's static.

[MartinJohns]

Just pointing out that code like this is unsound in TypeScript. It's legible to provide a different incompatible constructor in your subclass, which would result in new this('hello world') to result in a runtime error.

class SomeChildClass extends SomeParentClass {
    constructor(n: number) {
        super('bla')

        // Causes: n.toExponential is not a function 
        n.toExponential();
    }
}

[sean256]

#5863 does in fact cover this