Downlevel Awaited

[Newbie012]

Typescript 4.5 has introduced the Awaited type, which recursively extracts the value inside a promise.

I wanted to contribute to this library, but I got confused about how to implement it since it calls itself:

type Awaited<T> =
    T extends null | undefined ? T : // special case for `null | undefined` when not in `--strictNullChecks` mode
        T extends object & { then(onfulfilled: infer F): any } ? // `await` only unwraps object types with a callable `then`. Non-object types are not unwrapped
            F extends ((value: infer V, ...args: any) => any) ? // if the argument to `then` is callable, extracts the first argument
                Awaited<V> : // recursively unwrap the value
                never : // the argument to `then` was not callable
        T; // non-object or non-thenable

I was able to downlevel it from:

export type K<T> = Awaited<Promise<T>>;

to

export type K<T> = T extends null | undefined ? T : T extends object & {
    then(onfulfilled: infer F): any;
} ? F extends ((value: infer V, ...args: any) => any) ? Awaited<V> : never : T;

As you can see, Awaited calls itself recursively. I tried to look for different similar types that call themselves recursively, but I found none.

I'll be happy to create a PR for this, but I'm not sure how to proceed from here.

You can see here my current attempt.

[Newbie012]

Not sure why I got confused. I was probably tired...

Closing this issue in favor of #79