Infer generic awaited type from Promise argument

Issue

I have a generic CacheBuilder factory witch takes a GetValue function in argument and returns an instance of Cache

type GetValue<T> = (() => T) | (() => Promise<T>)

type CacheBuilder = <T> (getValue: GetValue<T>, options?: BuildCacheOptions<T>) => Cache<T>

One thing to note is the getValue function can be both sync or async.

I'd like to be able to let Typescript infer T from getValue, but to always return the Awaited type.

Such as :

import { buildCache } from './cache'
// (buildCache is a CacheBuilder)

const getMyVal = () => Promise.resolve('foo')

// Inferred
const cache = buildCache(getMyVal)

My trouble is cache is inferred as Cache<Promise<string>> while I'd like it to be a Cache<string>

Solution

Instead of storing the type T in GetValue<T>, we can store GetValue directly like this, then get the return type of it and "await" it, but with types.

type CacheBuilder = <G extends GetValue<any>> (getValue: G, options?: BuildCacheOptions<Awaited<ReturnType<G>>>) => MyCache<Awaited<ReturnType<G>>>

Playground

Awaited type

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EDIT after accept:

Previous concept works perfectly, but here's a cleaner version, with extracted type as generic too (which allows the simple use of T inside function implementation)

type CacheBuilder = <G extends GetValue<any>, T extends Awaited<ReturnType<G>>> (getValue: G, options?: BuildCacheOptions<T>) => MyCache<T>

Playground

Answered By - kelly