feat: support generated locale type

packages/core-base/src/runtime.ts

@@ -11,6 +11,7 @@ import {
 } from '@intlify/shared'
 import { HelperNameMap } from '@intlify/message-compiler'
 import { Path } from './resolver'
+import { IsNever } from './types'
 
 type ExtractToStringKey<T> = Extract<keyof T, 'toString'>
 type ExtractToStringFunction<T> = T[ExtractToStringKey<T>]
@@ -21,8 +22,38 @@ type StringConvertable<T> = ExtractToStringKey<T> extends never
 	  ? T
 	  : unknown
 
+/**
+ *
+ * The interface used for narrowing types using generated types.
+ *
+ * @remarks
+ *
+ * The type generated by 3rd party (e.g. nuxt/i18n)
+ *
+ * @example
+ * ```ts
+ * // generated-i18n-types.d.ts (`.d.ts` file at your app)
+ *
+ * declare module '@intlify/core' {
+ *   interface GeneratedTypeConfig {
+ *     locale: "en" | "ja"
+ *   }
+ * }
+ * ```
+ */
+export interface GeneratedTypeConfig {}
+
+/**
+ * Generated locale which resolves to `never` if left unset
+ */
+export type GeneratedLocale =
+  GeneratedTypeConfig extends Record<'locale', infer CustomLocale>
+    ? CustomLocale
+    : never
+
 /** @VueI18nGeneral */
-export type Locale = string
+export type Locale =
+  IsNever<GeneratedLocale> extends true ? string : GeneratedLocale
 
 /** @VueI18nGeneral */
 // prettier-ignore

packages/petite-vue-i18n/src/index.ts

@@ -42,7 +42,8 @@ export {
   MessageCompilerContext,
   CompileError,
   MessageContext,
-  RemovedIndexResources
+  RemovedIndexResources,
+  GeneratedTypeConfig
 } from '@intlify/core-base'
 export {
   VueMessageType,

packages/vue-i18n-core/src/composer.ts

@@ -109,7 +109,8 @@ import type {
   IsEmptyObject,
   CoreMissingType,
   JsonPaths,
-  TranslationsPaths
+  TranslationsPaths,
+  GeneratedLocale
 } from '@intlify/core-base'
 import type { VueDevToolsEmitter } from '@intlify/devtools-types'
 
@@ -1274,11 +1275,13 @@ export interface Composer<
     | PickupLocales<NonNullable<Messages>>
     | PickupLocales<NonNullable<DateTimeFormats>>
     | PickupLocales<NonNullable<NumberFormats>>,
-  Locales = OptionLocale extends Locale
-    ? IsNever<ResourceLocales> extends true
-      ? Locale
-      : ResourceLocales
-    : OptionLocale | ResourceLocales
+  Locales = Locale extends GeneratedLocale
+    ? GeneratedLocale
+    : OptionLocale extends Locale
+      ? IsNever<ResourceLocales> extends true
+        ? Locale
+        : ResourceLocales
+      : OptionLocale | ResourceLocales
 > extends ComposerCustom {
   /**
    * @remarks

packages/vue-i18n-core/src/legacy.ts

@@ -46,7 +46,8 @@ import type {
   PickupPaths,
   PickupFormatPathKeys,
   IsEmptyObject,
-  IsNever
+  IsNever,
+  GeneratedLocale
 } from '@intlify/core-base'
 import type { VueDevToolsEmitter } from '@intlify/devtools-types'
 import type {
@@ -1047,11 +1048,13 @@ export interface VueI18n<
     | PickupLocales<NonNullable<Messages>>
     | PickupLocales<NonNullable<DateTimeFormats>>
     | PickupLocales<NonNullable<NumberFormats>>,
-  Locales = OptionLocale extends string
-    ? [ResourceLocales] extends [never]
-      ? Locale
-      : ResourceLocales
-    : OptionLocale | ResourceLocales,
+  Locales = Locale extends GeneratedLocale
+    ? GeneratedLocale
+    : OptionLocale extends string
+      ? [ResourceLocales] extends [never]
+        ? Locale
+        : ResourceLocales
+      : OptionLocale | ResourceLocales,
   Composition extends Composer<
     Messages,
     DateTimeFormats,

packages/vue-i18n/src/index.ts

@@ -52,7 +52,8 @@ export {
   MessageCompilerContext,
   CompileError,
   MessageContext,
-  RemovedIndexResources
+  RemovedIndexResources,
+  GeneratedTypeConfig
 } from '@intlify/core-base'
 export {
   VueMessageType,