[strict-route-types] Enforce common React Component return types in App Router (#87389)
Flagged behind `experimental.strictRouteTypes`.
Most notably this disallows `void`. We generally treat `void` as "forgot to add return" in React e.g.
```tsx
async function Page() {
<div />;
}
```
which is a common beginner mistake. The downside is that this breaks unconditional `notFound()` or `redirect()` in Next.js. Those function throw but even with `never` as a return type, [TypeScript will not infer that this causes the caller to return `never` as well](https://github.com/microsoft/TypeScript/issues/26537).
```tsx
// implicit `void` return type instead of `never` due to calling `notFound(): never`
function Page() {
notFound()
}
```
The fix is to explicitly annotate unconditional `notFound` usage:
```tsx
function Page(): never {
notFound()
}
```
Conditional usage would already have another `return` statement. If that's missing still, we'd consider this a mistake. You can always do `return` without a value to fix the type-issue.
Pages in Pages Router are not affected. Tightening types for Pages Router is out-of-scope for this project.
Closes https://linear.app/vercel/issue/NXT-127/
next.js
0f2bbc9a - [strict-route-types] Enforce common React Component return types in App Router (#87389)
