A "use server" file can only export async functions' error when exporting non-async/cache functions

[RiodeJaneiroo]

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.3.0: Wed Dec 20 21:30:27 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T8103
  Available memory (MB): 8192
  Available CPU cores: 8
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: 1.22.21
  pnpm: 8.15.4
Relevant Packages:
  next: 14.2.0-canary.2 // Latest available version is detected (14.2.0-canary.2).
  eslint-config-next: 14.1.2
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.3.3
Next.js Config:
  output: standalone

Which example does this report relate to?

none

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Describe the Bug

When using the "use server" directive at the top of a file in a Next.js application, if the file exports a non-async function or a React cache function (e.g., React.cache), it throws the following error:

Error: A "use server" file can only export async functions.

Expected Behavior

The expected behavior is to allow exporting non-async functions and React cache functions from a file with the "use server" directive without any errors.

To Reproduce

StackBlitz

[chungweileong94]

The issue is caused by the stricter use server check in the recent 14.1.2 update, it seems to me NextJS is performing syntax-level checking which might raise some false errors. It actually breaks my server action builder package.

Repro: https://github.com/chungweileong94/next-use-server-bug

[ceIia]

yeah, i had to make sure i used the export async function(return somepromise) syntax, as export const = async () => { await ... } would not work

[chungweileong94]

It seems like NextJS is doing action.constructor.name !== 'AsyncFunction' check, however, if the action is created via a library, it will get transpiled into Promise<...> instead of action.constructor.name !== 'AsyncFunction'. The only workaround now is to create a async function wrapper around, which is kinda annoying.

next.js/packages/next/src/build/webpack/loaders/next-flight-loader/action-validate.ts

Line 16 in 7f7943b

// Note: if it's from library code with `async` being transpiled to returning a promise,

[bduff9]

Its worth noting that this currently breaks the example shown in the documentation around using React.cache currently: https://nextjs.org/docs/app/building-your-application/data-fetching/fetching-caching-and-revalidating#fetching-data-on-the-server-with-third-party-libraries

[shuding]

This is a duplicate of #62860 - please give #62860 (comment) and #62821 a read. We'll keep everyone updated there.

Basically it does not make sense to use React.cache together with Server Actions and it will not cache anything.

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.