Check for type of route handler returned value at build time (via the TS plugin) and at runtime

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

@@ -69,7 +69,8 @@ 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 = (
   /**
@@ -81,7 +82,7 @@ export type AppRouteHandlerFn = (
    * dynamic route).
    */
   ctx: AppRouteHandlerFnContext
-) => Promise<Response> | Response
+) => unknown
 
 /**
  * AppRouteHandlers describes the handlers for app routes that is provided by
@@ -364,6 +365,11 @@ export class AppRouteRouteModule extends RouteModule<
                     const res = await handler(wrappedRequest, {
                       params: context.params,
                     })
+                    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
 

test/e2e/app-dir/app-routes/app-custom-routes.test.ts

@@ -629,5 +629,18 @@ createNextDescribe(
         })
       })
     }
+
+    describe('no response returned', () => {
+      it('should print an error when no response is returned', async () => {
+        await next.fetch('/no-response', { method: 'POST' })
+
+        await check(() => {
+          expect(next.cliOutput).toMatch(
+            /No response is returned from route handler '.+\/route\.ts'\. Ensure you return a `Response` or a `NextResponse` in all branches of your handler\./
+          )
+          return 'yes'
+        }, 'yes')
+      })
+    })
   }
 )

test/e2e/app-dir/app-routes/app/no-response/route.ts

@@ -0,0 +1 @@
+export function POST() {}