From 6bbb52edfe3e969b4a80dc2d32c961303bb8c848 Mon Sep 17 00:00:00 2001
From: Vu Van Dung <me@joulev.dev>
Date: Thu, 17 Nov 2022 06:25:20 +0800
Subject: [PATCH] Allow generateStaticParams to be a synchronous function in
 app directory (#42942)

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## Summary

In `appDir`, currerntly `generateStaticParams` must be an `async`
function, even if no data fetching or asynchronous operations are
involved. For example, with this

```ts
// app/[slug]/page.tsx
export async function generateStaticParams() {
  return [{ slug: "Hello" }];
}
```

if I remove the `async` keyword, `next build` type checking will fail
with a rather unclear error message

```
Type error: Page "app/[slug]/page.tsx" does not match the required types of a Next.js Page.
```

However `next dev` still works fine, and after applying the type change
in this PR, `next build` and `next start` will also work fine.

Considering that `getStaticPaths` can be synchronous, this requirement
of `async` is pretty confusing. Many people have reported this type
error for not marking `generateStaticParams` as `async` (me included).

This PR lifts the restriction and allows `generateStaticParams` to be
synchronous. (If it's intentional that `generateStaticParams` must be
asynchronous, feel free to close this PR, but I don't think that
restriction is a good idea...)

Since I cannot find any test cases on the type checking process, I'm a
bit unsure in how to write a test case for this... or if a test case is
even necessary at all.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
---
 packages/next/build/webpack/plugins/flight-types-plugin.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/next/build/webpack/plugins/flight-types-plugin.ts b/packages/next/build/webpack/plugins/flight-types-plugin.ts
index 54d31b078a7453c..15371d5337a2b75 100644
--- a/packages/next/build/webpack/plugins/flight-types-plugin.ts
+++ b/packages/next/build/webpack/plugins/flight-types-plugin.ts
@@ -45,7 +45,7 @@ interface IEntry {
       : `default: PageComponent`
   }
   config?: {}
-  generateStaticParams?: (params?: PageParams) => Promise<any[]>
+  generateStaticParams?: (params?: PageParams) => any[] | Promise<any[]>
   revalidate?: RevalidateRange<TEntry> | false
   dynamic?: 'auto' | 'force-dynamic' | 'error' | 'force-static'
   dynamicParams?: boolean