Awaited return type of generic parameter not assignable to itself

[wbt]

Bug Report

🔎 Search Terms

generic function parameter 2322 return type

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about functions and common bugs that aren't bugs
• I was unable to test this on versions before 4.5 because the Awaited keyword didn't exist.

⏯ Playground Link

Playground link with relevant code

💻 Code

type FetcherFunction = (
    identifier: number
) => Promise<string>; //fixed 'string' not tied to parameter types is part of simplification for example
type demoFine1 = ReturnType<FetcherFunction>; // Promise<string>
type demoFine2 = Awaited<ReturnType<FetcherFunction>>; //string
async function getterDemo<
    F extends FetcherFunction = FetcherFunction
>(
    fetcherFn: F,
    identifier: number,
) : Promise<Awaited<ReturnType<F>>> {
    let demo3: F;
    let result = await fetcherFn(identifier); //result is a string
    return result; //ERROR TS2322: Type 'string' is not assignable to type 'Awaited<ReturnType<F>>'
    //In the motivating example, lots more is done with the result and the return type is a type derived from it,
    //but the simpler example seems like it shouldn't be throwing an error: whether Awaited<ReturnType<F> is a
    //simple string or something more complex, that is what the promise returned by this async function resolves to.
}

🙁 Actual behavior

Error as noted.

🙂 Expected behavior

No TS2322 error; the above code works fine. The awaited return type of fetcherFn should be assignable to the return (resolve) value, which is the awaited return type of that function type.

[MartinJohns]

The issue is the deferred resolving of conditional types involving unbound type parameters. Plenty of issues about that.

Get rid of ReturnType<> and it will work fine.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.