improvement: allow custom block useI18n on Legacy API mode (#1022)

resolve #1020

Author: kazupon

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

@@ -71,7 +71,7 @@ export const errorMessages: { [code: number]: string } = {
   [I18nErrorCodes.BRIDGE_SUPPORT_VUE_2_ONLY]:
     'vue-i18n-bridge support Vue 2.x only',
   [I18nErrorCodes.MUST_DEFINE_I18N_OPTION_IN_ALLOW_COMPOSITION]:
-    'Must define ‘i18n’ option in Composition API with using local scope in Legacy API mode',
+    'Must define ‘i18n’ option or custom block in Composition API with using local scope in Legacy API mode',
   [I18nErrorCodes.NOT_AVAILABLE_COMPOSITION_IN_LEGACY]:
     'Not available Compostion API in Legacy API mode. Please make sure that the legacy API mode is working properly'
 }

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

@@ -1099,7 +1099,11 @@ function useI18nForLegacy(
   const isLocale = scope === 'local'
   const _composer = shallowRef<Composer | null>(null)
 
-  if (isLocale && instance.proxy && !instance.proxy.$options.i18n) {
+  if (
+    isLocale &&
+    instance.proxy &&
+    !(instance.proxy.$options.i18n || instance.proxy.$options.__i18n)
+  ) {
     throw createI18nError(
       I18nErrorCodes.MUST_DEFINE_I18N_OPTION_IN_ALLOW_COMPOSITION
     )

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

@@ -525,6 +525,45 @@ describe('useI18n', () => {
           expect(html()).toEqual('<p>en:world!</p>')
         })
 
+        test('use custom block', async () => {
+          const i18n = createI18n({
+            allowComposition: true,
+            locale: 'ja',
+            messages: {
+              en: {
+                hello: 'hello!'
+              },
+              ja: {}
+            }
+          })
+
+          const App = defineComponent({
+            setup() {
+              const instance = getCurrentInstance()
+              if (instance == null) {
+                throw new Error()
+              }
+              const options = instance.type as ComponentOptions
+              options.__i18n = [
+                {
+                  locale: 'ja',
+                  resource: {
+                    hello: 'こんにちは!'
+                  }
+                }
+              ]
+              const { locale, t } = useI18n({
+                inheritLocale: true,
+                useScope: 'local'
+              })
+              return { locale, t }
+            },
+            template: `<p>{{ locale }}:{{ t('hello') }}</p>`
+          })
+          const { html } = await mount(App, i18n)
+          expect(html()).toEqual('<p>ja:こんにちは!</p>')
+        })
+
         test('not defined i18n option in local scope', async () => {
           const i18n = createI18n({
             allowComposition: true,