Broken overloads with type inference on callbacks

[millsp]

TS fails to remember the overloaded types and picks the last one by default, breaking the overload usability (in practice) when using callbacks. Let's take a simple example to make this more clear:

EDIT: Which seems to be a limitation of the typeof operator.

TypeScript Version: 3.7.0-dev.20190905

Search Terms: callback, inference, overloads

Code

declare function fn(a: string)
declare function fn(a: string, b: number)
declare function fn(a: object, b: number, c: object)

declare function promisify<Fn extends (...args: any) => any>(fn: Fn): (...args: Parameters<Fn>) => Promise<ReturnType<Fn>>

const promisedFn = promisify(fn)

const test0 = promisedFn('hello') // error
// only allows fn(a: object, b: number, c: object)

Expected behavior:
There shouldn't be any error. Maybe we could treat overloads as unions?

Actual behavior:
TypeScript picks that last overload only

Playground Link:

Related Issues:

[millsp]

Closing in favor of #598