Fix return type of middleware req.cookies.get() (#36872)

Follow up to #36478 

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

packages/next/server/web/spec-extension/cookies.ts

@@ -35,7 +35,7 @@ const deserializeCookie = (input: Request | Response): string[] => {
 
 const serializeCookie = (input: string[]) => input.join(', ')
 
-export class Cookies extends Map<string, any> {
+export class Cookies extends Map<string, string> {
   constructor(input?: string | null) {
     const parsedInput = typeof input === 'string' ? cookie.parse(input) : {}
     super(Object.entries(parsedInput))
@@ -62,6 +62,13 @@ export class NextCookies extends Cookies {
   set = (...args: Parameters<Cookies['set']>) => {
     const isAlreadyAdded = super.has(args[0])
     const store = super.set(...args)
+    const currentCookie = store.get(args[0])
+
+    if (typeof currentCookie !== 'string') {
+      throw new Error(
+        `Invariant: failed to generate cookie for ${JSON.stringify(args)}`
+      )
+    }
 
     if (isAlreadyAdded) {
       const setCookie = serializeCookie(
@@ -73,13 +80,13 @@ export class NextCookies extends Cookies {
       if (setCookie) {
         this.response.headers.set(
           'set-cookie',
-          [store.get(args[0]), setCookie].join(', ')
+          [currentCookie, setCookie].join(', ')
         )
       } else {
-        this.response.headers.set('set-cookie', store.get(args[0]))
+        this.response.headers.set('set-cookie', currentCookie)
       }
     } else {
-      this.response.headers.append('set-cookie', store.get(args[0]))
+      this.response.headers.append('set-cookie', currentCookie)
     }
 
     return store

test/production/middleware-typescript/app/pages/rewrites/_middleware.ts

@@ -10,6 +10,9 @@ export const middleware: NextMiddleware = async function (request) {
       const response = NextResponse.rewrite(`/rewrites/${bucket}`)
       response.cookies.set('bucket', bucket, { maxAge: 10 })
       return response
+    } else {
+      // check that `bucket` is type "string", not "any"
+      bucket.toUpperCase()
     }
 
     return NextResponse.rewrite(`/rewrites/${bucket}`)