As mentioned in the new-added error messages, and the [linked
resources](https://react.dev/reference/react/use-server#:~:text=Because%20the%20underlying%20network%20calls%20are%20always%20asynchronous%2C%20%27use%20server%27%20can%20only%20be%20used%20on%20async%20functions.):
> Because the underlying network calls are always asynchronous, 'use
server' can only be used on async functions.
> https://react.dev/reference/react/use-server
It's a requirement that only async functions are allowed to be exported
and annotated with `'use server'`. Currently, we already have compiler
check so this will already error:
```js
'use server'
export function foo () {} // missing async
```
However, since exported values can be very dynamic the compiler can't
catch all mistakes like that. We also have a runtime check for all
exports in a `'use server'` function, but it only covers `typeof value
=== 'function'`.
This PR adds a stricter check for "use server" annotated values to also
make sure they're async functions (`value.constructor.name ===
'AsyncFunction'`).
That said, there are still cases like synchronously returning a promise
to make a function "async", but it's still very different by definition.
For example:
```js
const f = async () => { throw 1; return 1 }
const g = () => { throw 1; return Promise.resolve(1) }
```
Where `g()` can be synchronously caught (`try { g() } catch {}`) but
`f()` can't even if they have the same types. If we allow `g` to be a
Server Action, this behavior is no longer always true but depending on
where it's called (server or client).
Closes#62727.
