I'm trying to create something similar to native ReturnType type. In comparison to the later, it should also consider generic types of arguments.

I guess the idea will be clear from the code below.

If it can be done now - I'd be happy to receive some links. Otherways it'd be great to know if there are any plans to cover that behavior. In case it could be achieved by another not-implemented feature I'll be glad for the link to an issue to be able to track its progress.

TypeScript Version: 2.9.2

Search Terms: infer function return type

Code

// it works with actual code:
function identity<T>(a: T) {
    return a;
}

const one = identity(1); // typeof one === 1 # everything is great

function call<A, R>(arg: A, fn: Func<A, R>): R {
    return fn(arg);
}

const two = call(2 as 2, i => i) // typeof two === 2 # everything is great

// but won't work with types
type Func<A, R> = (arg: A) => R;
// ReturnTypeByArg is inspired by `ReturnType` from TS 2.8.*
type ReturnTypeByArg<T, F extends Func<T, any>> = F extends Func<T, infer R> ? R : any;

type Identity = <T>(a: T) => T;
type Result = ReturnTypeByArg<4, Identity>; // Result === {} # not what I expect

Expected behavior:
I expect TS to be able to infer such return type

Actual behavior:
Return type is {}

Playground Link: link

Related Issues: