Check for type of route handler returned value at build time (via the…

… TS plugin) and at runtime (#51394)

### What?

Fixes #51130. Before this PR, the package assumes that route handlers return a `Response` which is not necessarily the case.

The linked issue specified three suggestions to resolve this

1. Return a default 200 response
2. Throw a better error message
3. or both

~~In this issue I implemented (3), except that it is a warning and not an error. Do tell if the team wants to follow a different approach, as it is not too hard to change this.~~

This PR implements (2).

### How?

The returned value of the handler is checked at runtime to ensure it is actually a `Response` instance.

The return type `AppRouteHandlerFn` is also modified to `unknown` to avoid similar assumptions elsewhere.

The TS plugin is also modified to check for the return type during build time.



Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

packages/next/src/build/webpack/plugins/next-types-plugin/index.ts

@@ -107,6 +107,19 @@ if ('${method}' in entry) {
       '${method}'
     >
   >()
+  checkFields<
+    Diff<
+      {
+        __tag__: '${method}',
+        __return_type__: Response | Promise<Response>
+      },
+      {
+        __tag__: '${method}',
+        __return_type__: ReturnType<MaybeField<TEntry, '${method}'>>
+      },
+      '${method}'
+    >
+  >()
 }
 `
       ).join('')

packages/next/src/server/future/route-modules/app-route/module.ts

@@ -70,7 +70,8 @@ type AppRouteHandlerFnContext = {
 }
 
 /**
- * Handler function for app routes.
+ * Handler function for app routes. If a non-Response value is returned, an error
+ * will be thrown.
  */
 export type AppRouteHandlerFn = (
   /**
@@ -82,7 +83,7 @@ export type AppRouteHandlerFn = (
    * dynamic route).
    */
   ctx: AppRouteHandlerFnContext
-) => Promise<Response> | Response
+) => unknown
 
 /**
  * AppRouteHandlers describes the handlers for app routes that is provided by
@@ -368,6 +369,11 @@ export class AppRouteRouteModule extends RouteModule<
                         ? parsedUrlQueryToParams(context.params)
                         : undefined,
                     })
+                    if (!(res instanceof Response)) {
+                      throw new Error(
+                        `No response is returned from route handler '${this.resolvedPagePath}'. Ensure you return a \`Response\` or a \`NextResponse\` in all branches of your handler.`
+                      )
+                    }
                     ;(context.staticGenerationContext as any).fetchMetrics =
                       staticGenerationStore.fetchMetrics