Defer generic awaited type

[jablko]

async function f<T>(x: T) {
    return x;
}
const expected: Promise<number> = f(undefined as Promise<number>);

Before

!!! error TS2322: Type 'Promise<Promise<number>>' is not assignable to type 'Promise<number>'.
!!! error TS2322:   Type 'Promise<number>' is not assignable to type 'number'.

After

getAwaitedType() returns the awaited type if it's known, or Awaited<T> if it's generic.

Fixes #27711

This proposal excludes non-A+ promises, which has the benefit of not requiring a recursive solution or a new kind of type, since an A+ promise can't resolve to another promise.

@typescript-bot test this
@typescript-bot run dt
@typescript-bot user test this

[orta]

@typescript-bot test this
@typescript-bot user test this
@typescript-bot run dt

[typescript-bot]

Heya @orta, I've started to run the extended test suite on this PR at 3bab709. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

Heya @orta, I've started to run the parallelized Definitely Typed test suite on this PR at 3bab709. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

Heya @orta, I've started to run the parallelized community code test suite on this PR at 3bab709. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

[rbuckton]

This was superseded by #45350

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #27711. If you can get it accepted, this PR will have a better chance of being reviewed.