Inferring "this" from arrow function is {}

TypeScript Version: Version 3.6.0-dev.20190601 Search Terms: This, Arrow, Functions, Infer, Inference, Empty, Object, Incorrect, Any

Code

type FunctionThisType<T extends (...args: any[]) => any> =
  T extends (this: infer R, ...args: any[]) => any ? R : any

let fn = () => {}
let value: FunctionThisType<typeof fn> = "wrong"

Expected behavior:

error TS2322: Type '"wrong"' is not assignable to type 'typeof globalThis'.

Found 1 error.

Actual behavior:

No errors found.

Playground Link: Playground

Related Issues: None found.

[RyanCavanaugh]

Concrete example of desirable behavior:

function callWithThis<T extends Function>(fn: T, ths: ThisType<T>) {
    fn.call(ths);
}

const arrow = () => 0;
// Should be OK
callWithThis(arrow, null);

[jamiebuilds]

@kitsonk Yes I understand how arrow functions work and I understand how TypeScript inference works. I understand that a this type in an arrow function doesn’t make much sense. However, the bug that I am reporting is that TypeScript does produce a type for this in an arrow function, and that type is incorrectly.

TypeScript could (and in fact does appear to in some cases) store the this context for an arrow function from its definition, and then it can later use that this type to fix this inference bug that I am reporting.

If TypeScript wants to change that type to any or something, that would also be fine, but right now I am able to produce correct JavaScript code that TypeScript reports as incorrect and this is the underlying cause.

And I don’t need you to explain JavaScript to me, I worked on Babel, TC39, and Flow, I gots it