Hotfix: Temporarily disable static type checking for route handler return type

[joulev]

Why?

#51394 added static type checking for route handlers return type, so all route handlers must return Response | Promise<Response>.

But the checker cannot detect when the function always throws, for example

export function GET() {
  throw new Error(); // can be notFound(), redirect()
}

because GET is inferred as () => void, not as () => never, adding | never to the type checker can't fix it.

I can't find a way to get this to work (see the "Notes" section below for more details), so this PR disables that entire type checking so route handlers like the above can pass. Hopefully this change is temporary.

How?

Remove the part where the TS plugin checks for the return type of route handlers.

Notes

Without this PR, the following still works, because the return type is made known to TypeScript:

// Explicit return type
export function GET(): Response {
  throw new Error();
}

Allowing never in the TS plugin

-      __return_type__: Response | Promise<Response>
+      __return_type__: Response | Promise<Response> | never

will also make the following work:

// Arrow function
export const GET = () => {
  throw new Error();
}

because GET is inferred to return never. However, using regular function declaration won't work because it is inferred to return void (see microsoft/TypeScript#8767 and microsoft/TypeScript#16608). I can't figure out how to get type checking to work without requiring users to either use arrow functions or explicitly define the return type.

Interested people can go to this minimal TypeScript example and try to fix.

[ijjk]

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[orca-security-us]

Orca Security Scan Summary

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[joulev]

Assuming TypeScript doesn't fix the inference bug (microsoft/TypeScript#16608 (comment)), which is very likely, I think we either have to disable static build-time type-checking (this PR) or force users to use return types in this case (requiring code updates for some users). Both are bad but I can't see a third way.

[joulev]

Closing in favour of #55849.