Method with "(this: undefined)" cannot be called directly

[ChrisGrohHS]

Bug Report

🔎 Search Terms

"this: undefined"

🕗 Version & Regression Information

I tried it with Nightly and 3.3.3333.

• This is the behavior in every version I tried including Nightly, and I reviewed the FAQ for entries about "this"

⏯ Playground Link

Playground link with relevant code

💻 Code

type UndefinedThisCallback = (this: undefined) => number;
const callback: UndefinedThisCallback = () => 5;
callback();// error
callback.call(undefined);// workaround

🙁 Actual behavior

Unable to call the method without using .call.

The 'this' context of type 'void' is not assignable to method's 'this' of type 'undefined'.

With nothing on the left side of the call, undefined should be what is passed to the callback for this.

🙂 Expected behavior

Able to call the method normally.

Why don't you use (this: void)?

When writing a library, one easy mistake is to expose internal objects via this. This can mean that changes you expected to be non-breaking can end up breaking consumers. By typing all public API functions using this: undefined, type checking gives an error to keep us from leaking internal objects.

Playground Link

type VoidThisCallback = (this: void) => number;

class MyInternalClass {
    constructor(private callback: VoidThisCallback) {
    }
    runCallback() {
        this.callback();// leaks internal object
    }
}

[MartinJohns]

Wouldn't your issue with (this: void) be solved with #44513?

[ChrisGrohHS]

Yes, it would be solved by #44513. After reading about void returning functions in the FAQ, I thought that this was also purposeful for this as well.