feat: configurate custom property $i18n type (#2093)

Author: BobbieGoede

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

@@ -99,7 +99,12 @@ export type {
 } from './legacy'
 export { initDev, initFeatureFlags, VERSION } from './misc'
 export type { I18nPluginOptions } from './plugin'
-export type { Disposer } from './types'
+export type {
+  Disposer,
+  GeneratedInstanceType,
+  GeneratedTypeConfig,
+  VueI18nInstance
+} from './types'
 
 export type {
   IsEmptyObject,

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

@@ -1 +1,45 @@
+import type { IsNever } from '@intlify/core-base'
+import type { ExportedGlobalComposer } from './i18n'
+import type { VueI18n } from './legacy'
+
 export type Disposer = () => void
+
+/**
+ *
+ * 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/vue-i18n-core' {
+ *   interface GeneratedTypeConfig {
+ *     legacy: false
+ *   }
+ * }
+ * ```
+ */
+
+export interface GeneratedTypeConfig {}
+
+/**
+ * Narrowed i18n instance type based on `GeneratedTypeConfig['legacy']`
+ *
+ * - `never` (unset) resolves to `VueI18n | ExportedGlobalComposer`
+ * - `true` resolves to `VueI18n`
+ * - `false` resolves to `ExportedGlobalComposer`
+ */
+export type GeneratedInstanceType =
+  GeneratedTypeConfig extends Record<'legacy', infer Legacy> ? Legacy : never
+
+/** @VueI18nGeneral */
+export type VueI18nInstance =
+  IsNever<GeneratedInstanceType> extends true
+    ? VueI18n | ExportedGlobalComposer
+    : GeneratedInstanceType extends true
+      ? VueI18n
+      : ExportedGlobalComposer

packages/vue-i18n/src/index.ts

@@ -110,6 +110,7 @@ export type {
   VueI18n,
   VueI18nDateTimeFormatting,
   VueI18nExtender,
+  VueI18nInstance,
   VueI18nNumberFormatting,
   VueI18nOptions,
   VueI18nResolveLocaleMessageTranslation,

packages/vue-i18n/src/vue.d.ts

@@ -16,13 +16,12 @@ import type {
   DateTimeFormatResult,
   DefineDateTimeFormat,
   DefineLocaleMessage,
-  ExportedGlobalComposer,
   NumberFormat,
   NumberFormatResult,
   RemovedIndexResources,
   TranslateResult,
   Translation,
-  VueI18n,
+  VueI18nInstance,
   VueI18nOptions,
   VueMessageType
 } from '@intlify/vue-i18n-core'
@@ -72,7 +71,7 @@ declare module 'vue' {
      * The locales, locale messages, and other resources managed by the instance referenced by this property are valid as global scope.
      * If the `i18n` component custom option is not specified, it's the same as the VueI18n instance that can be referenced by the i18n instance {@link I18n.global | global} property.
      */
-    $i18n: VueI18n | ExportedGlobalComposer
+    $i18n: VueI18nInstance
     /**
      * Locale message translation
      *