Add err.sh for i18n config errors (#18425)

This adds an err.sh with info that can help when tracking down errors from invalid i18n config in next.config.js

errors/invalid-i18n-config.md

+# Invalid i18n config
+
+#### Why This Error
+
+In your `next.config.js` file you provided an invalid config for the `i18n` field.
+
+#### Possible Ways to Fix It
+
+Make sure your `i18n` field follows the allowed config shape and values:
+
+```js
+module.exports = {
+  i18n: {
+    // These are all the locales you want to support in
+    // your application
+    locales: ['en-US', 'es', 'fr', 'nl-NL'],
+    // This is the default locale you want to be used when visiting
+    // a non-locale prefixed path e.g. `/hello`
+    defaultLocale: 'en-US',
+    // This is a list of locale domains and the default locale they
+    // should handle (these are only required when setting up domain routing)
+    domains: [
+      {
+        domain: 'example.com',
+        defaultLocale: 'en-US',
+        // other locales that should be handled on this domain
+        locales: ['es'],
+      },
+      {
+        domain: 'example.nl',
+        defaultLocale: 'nl-NL',
+      },
+      {
+        domain: 'example.fr',
+        defaultLocale: 'fr',
+      },
+    ],
+  },
+}
+```
+
+### Useful Links
+
+- [Internationalized Routing Documentation](https://nextjs.org/docs/advanced-features/i18n-routing)

packages/next/next-server/server/config.ts

@@ -315,24 +315,28 @@ function assignDefaults(userConfig: { [key: string]: any }) {
     const i18nType = typeof i18n
 
     if (i18nType !== 'object') {
-      throw new Error(`Specified i18n should be an object received ${i18nType}`)
+      throw new Error(
+        `Specified i18n should be an object received ${i18nType}.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
+      )
     }
 
     if (!Array.isArray(i18n.locales)) {
       throw new Error(
-        `Specified i18n.locales should be an Array received ${typeof i18n.locales}`
+        `Specified i18n.locales should be an Array received ${typeof i18n.locales}.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
       )
     }
 
     const defaultLocaleType = typeof i18n.defaultLocale
 
     if (!i18n.defaultLocale || defaultLocaleType !== 'string') {
-      throw new Error(`Specified i18n.defaultLocale should be a string`)
+      throw new Error(
+        `Specified i18n.defaultLocale should be a string.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
+      )
     }
 
     if (typeof i18n.domains !== 'undefined' && !Array.isArray(i18n.domains)) {
       throw new Error(
-        `Specified i18n.domains must be an array of domain objects e.g. [ { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] } ] received ${typeof i18n.domains}`
+        `Specified i18n.domains must be an array of domain objects e.g. [ { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] } ] received ${typeof i18n.domains}.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
       )
     }
 
@@ -370,14 +374,14 @@ function assignDefaults(userConfig: { [key: string]: any }) {
             .map((item: any) => JSON.stringify(item))
             .join(
               '\n'
-            )}\n\ndomains value must follow format { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] }`
+            )}\n\ndomains value must follow format { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] }.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
         )
       }
     }
 
     if (!Array.isArray(i18n.locales)) {
       throw new Error(
-        `Specified i18n.locales must be an array of locale strings e.g. ["en-US", "nl-NL"] received ${typeof i18n.locales}`
+        `Specified i18n.locales must be an array of locale strings e.g. ["en-US", "nl-NL"] received ${typeof i18n.locales}.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
       )
     }
 
@@ -387,14 +391,18 @@ function assignDefaults(userConfig: { [key: string]: any }) {
 
     if (invalidLocales.length > 0) {
       throw new Error(
-        `Specified i18n.locales contains invalid values, locales must be valid locale tags provided as strings e.g. "en-US".\n` +
+        `Specified i18n.locales contains invalid values (${invalidLocales
+          .map(String)
+          .join(
+            ', '
+          )}), locales must be valid locale tags provided as strings e.g. "en-US".\n` +
           `See here for list of valid language sub-tags: http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry`
       )
     }
 
     if (!i18n.locales.includes(i18n.defaultLocale)) {
       throw new Error(
-        `Specified i18n.defaultLocale should be included in i18n.locales`
+        `Specified i18n.defaultLocale should be included in i18n.locales.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
       )
     }
 
@@ -411,7 +419,7 @@ function assignDefaults(userConfig: { [key: string]: any }) {
       localeDetectionType !== 'undefined'
     ) {
       throw new Error(
-        `Specified i18n.localeDetection should be undefined or a boolean received ${localeDetectionType}`
+        `Specified i18n.localeDetection should be undefined or a boolean received ${localeDetectionType}.\nSee more info here: https://err.sh/nextjs/invalid-i18n-config`
       )
     }
   }