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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)
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