Log dev error in Server Router (#45951)

Fixes NEXT-561

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) 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`](https://github.com/vercel/next.js/blob/canary/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)

Author: hanneslund

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -62,6 +62,7 @@ async function createAppRouteCode({
     import 'next/dist/server/node-polyfill-headers'
     
     export * as handlers from ${JSON.stringify(resolvedPagePath)}
+    export const resolvedPagePath = ${JSON.stringify(resolvedPagePath)}
 
     export { requestAsyncStorage } from 'next/dist/client/components/request-async-storage'
   `

packages/next/src/server/future/route-handlers/app-route-route-handler.ts

@@ -61,6 +61,11 @@ export type AppRouteModule = {
    * The exported async storage object for this worker/module.
    */
   requestAsyncStorage: RequestAsyncStorage
+
+  /**
+   * The absolute path to the module file
+   */
+  resolvedPagePath: string
 }
 
 /**
@@ -172,7 +177,26 @@ export class AppRouteRouteHandler implements RouteHandler<AppRouteRouteMatch> {
     if (!isHTTPMethod(req.method)) return handleBadRequestResponse
 
     // Pull out the handlers from the app route module.
-    const { handlers } = mod
+    const { handlers, resolvedPagePath } = mod
+
+    if (process.env.NODE_ENV !== 'production') {
+      // Print error in development if the exported handlers are in lowercase, only uppercase handlers are supported
+      for (const invalidMethodName of [
+        'get',
+        'head',
+        'options',
+        'post',
+        'put',
+        'delete',
+        'patch',
+      ]) {
+        if ((handlers as any)[invalidMethodName]) {
+          Log.error(
+            `Detected lowercase method '${invalidMethodName}' in '${resolvedPagePath}'. Export the uppercase '${invalidMethodName.toUpperCase()}' method name to fix this error.`
+          )
+        }
+      }
+    }
 
     // Check to see if the requested method is available.
     const handler: AppRouteHandlerFn | undefined = handlers[req.method]

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

@@ -14,7 +14,7 @@ createNextDescribe(
     // TODO-APP: enable after deploy support is added
     skipDeployment: true,
   },
-  ({ next }) => {
+  ({ next, isNextDev }) => {
     describe('basic fetch request with a response', () => {
       describe.each(['GET', 'POST', 'PUT', 'DELETE', 'PATCH'])(
         'made via a %s request',
@@ -311,5 +311,33 @@ createNextDescribe(
         )
       })
     })
+
+    if (isNextDev) {
+      describe('lowercase exports', () => {
+        it.each([
+          ['get'],
+          ['head'],
+          ['options'],
+          ['post'],
+          ['put'],
+          ['delete'],
+          ['patch'],
+        ])(
+          'should print an error when using lowercase %p in dev',
+          async (method: string) => {
+            await next.fetch('/lowercase/' + method)
+            expect(next.cliOutput).toContain(
+              `Detected lowercase method '${method}' in`
+            )
+            expect(next.cliOutput).toContain(
+              `Export the uppercase '${method.toUpperCase()}' method name to fix this error.`
+            )
+            expect(next.cliOutput).toMatch(
+              /Detected lowercase method '.+' in '.+\/route\.ts'\. Export the uppercase '.+' method name to fix this error\./
+            )
+          }
+        )
+      })
+    }
   }
 )

test/e2e/app-dir/app-routes/app/lowercase/delete/route.ts

@@ -0,0 +1,6 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+const del = handler
+export { del as delete }

test/e2e/app-dir/app-routes/app/lowercase/get/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const get = handler

test/e2e/app-dir/app-routes/app/lowercase/head/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const head = handler

test/e2e/app-dir/app-routes/app/lowercase/options/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const options = handler

test/e2e/app-dir/app-routes/app/lowercase/patch/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const patch = handler

test/e2e/app-dir/app-routes/app/lowercase/post/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const post = handler

test/e2e/app-dir/app-routes/app/lowercase/put/route.ts

@@ -0,0 +1,5 @@
+export const handler = async (): Promise<Response> => {
+  return new Response('hello, world')
+}
+
+export const put = handler