How to convince the typescript that unknown is possible in ReturnType?

Question

typescript

Hello.
Oil painting (and cheese):

function call<F extends () => unknown>(f: F): ReturnType<F> {
  return f()
}

It would seem, what is the problem? But alas, "TS2322: Type 'unknown' is not assignable to type 'ReturnType '" on the return line.

If you replace unknown with any other type, it also swears. And only the ungodly any , as well as the useless never in this situation, return without errors.

It looks like ReturnType forgets that it's actually the return value for f. What is he?

Answer 1

TypeScript has a number of problems with functional types in generics due to their variance
. Here is how the result will be similar, but works without problems:

function call<R>(f: () => R): R {
  return f()
}

Answer 2

The first thing that came to mind:
return f() as ReturnType<F>