fix(types): allow spyOn of "useI18n()" (#1815)

Author: pinguet62

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

@@ -742,7 +742,7 @@ export function useI18n<
   NonNullable<Options['messages']>,
   NonNullable<Options['datetimeFormats']>,
   NonNullable<Options['numberFormats']>,
-  Options['locale'] extends unknown ? string : Options['locale']
+  NonNullable<Options['locale']>
 >
 
 export function useI18n<

test-dts/vue-i18n/i18n.test-d.ts

@@ -10,6 +10,7 @@ import {
   useI18n,
   createI18n
 } from '../../packages/vue-i18n-core/src/i18n'
+import * as vueI18n from '../../packages/vue-i18n-core/src/i18n'
 import { SchemaParams, LocaleParams } from '../../packages/core-base/src'
 import { ResourceSchema, MyDatetimeScehma, MyNumberSchema } from '../schema'
 
@@ -505,3 +506,8 @@ expectType<{ zh: {}; 'ja-JP': { short: {} } }>(
   strictDirectI18n.datetimeFormats.value
 )
 expectType<{ ca: { currency: {} } }>(strictDirectI18n.numberFormats.value)
+
+// allow mocking
+vi.spyOn(vueI18n, 'useI18n').mockReturnValue(<ReturnType<typeof useI18n>>{
+  t: (key: string) => `{{${key}}}`
+})