Description
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
Awaitedkeyword 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.