feat(build): add validation to detect ambiguous app routes at build time

Introduces validateAppPaths() to catch route conflicts that were previously
going undetected until runtime. Routes with different dynamic segment names
(e.g., [slug] vs [modalSlug]) that normalize to the same pattern now trigger
clear build-time errors with actionable guidance.

This prevents subtle routing issues in parallel routes and interception routes
where structural equivalence wasn't being validated during the build process.

Author: wyattjoh

packages/next/errors.json

@@ -907,5 +907,22 @@
   "906": "Bindings not loaded yet, but they are being loaded, did you forget to await?",
   "907": "bindings not loaded yet.  Either call `loadBindings` to wait for them to be available or ensure that `installBindings` has already been called.",
   "908": "Invalid flags should be run as node detached-flush dev ./path-to/project [eventsFile]",
-  "909": "Failed to load SWC binary for %s/%s, see more info here: https://nextjs.org/docs/messages/failed-loading-swc"
+  "909": "Failed to load SWC binary for %s/%s, see more info here: https://nextjs.org/docs/messages/failed-loading-swc",
+  "910": "Optional route parameters are not yet supported (\"%s\") in route \"%s\".",
+  "911": "You cannot use both a required and optional catch-all route at the same level in route \"%s\".",
+  "912": "Ambiguous app routes detected:\\n\\n%s\\n\\nThese routes cannot be distinguished from each other when matching URLs. Please ensure that dynamic segments have unique patterns or use different static segments.",
+  "913": "Optional catch-all must be the last part of the URL in route \"%s\".",
+  "914": "You cannot have the same slug name \"%s\" repeat within a single dynamic path in route \"%s\".",
+  "915": "Detected a three-dot character ('…') at ('%s') in route \"%s\". Did you mean ('...')?",
+  "916": "Segment names may not start or end with extra brackets ('%s') in route \"%s\".",
+  "917": "You cannot define a route with the same specificity as an optional catch-all route (\"%s\" and \"%s/[[...%s]]\").",
+  "918": "Catch-all must be the last part of the URL in route \"%s\".",
+  "919": "You cannot have the slug names \"%s\" and \"%s\" differ only by non-word symbols within a single dynamic path in route \"%s\".",
+  "920": "Segment names may not start with erroneous periods ('%s') in route \"%s\".",
+  "921": "Detected a three-dot character ('…') in parameter \"%s\" in route \"%s\". Did you mean ('...')?",
+  "922": "Parameter names cannot be empty in route \"%s\".",
+  "923": "%s is being parsed as a normalized route, but it has a route group or parallel route segment.",
+  "924": "Invalid interception route: %s",
+  "925": "You cannot define a route with the same specificity as an optional catch-all route (\"%s\" and \"/[[...%s]]\").",
+  "926": "Optional route parameters are not yet supported (\"[%s]\") in route \"%s\"."
 }

packages/next/src/build/index.ts

@@ -230,6 +230,7 @@ import {
   writeValidatorFile,
 } from '../server/lib/router-utils/route-types-utils'
 import { Lockfile } from './lockfile'
+import { validateAppPaths } from './validate-app-paths'
 
 type Fallback = null | boolean | string
 
@@ -1406,6 +1407,13 @@ export default async function build(
       }
 
       const appPaths = Array.from(appPageKeys)
+
+      // Validate that the app paths are valid. This is currently duplicating
+      // the logic from packages/next/src/shared/lib/router/utils/sorted-routes.ts
+      // but is instead specifically focused on code that can be shared
+      // eventually with the development code.
+      validateAppPaths(appPaths)
+
       // Interception routes are modelled as beforeFiles rewrites
       rewrites.beforeFiles.push(
         ...generateInterceptionRoutesRewrites(appPaths, config.basePath)