Vue.extend does not accept a VueI18n class instance as i18n option

vue & vue-i18n version

2.4.2 & 7.0.5 (using dist from master in fiddle)

Reproduction Link

Minimal example http://jsfiddle.net/Ly55ew8m/2/

Our use case is not represented in that fiddle, see below.

Steps to reproduce

Passing a VueI18n instance to Vue.extend is ignored, only a plain old i18n options object works.

In fiddle just comment and un-comment the relevant lines

What is Expected?

If a class instance would work, we could keep (and export) a reference to the instance and import it in other parts of the application (like vuex actions) to control vue-i18n, i.e. locale switch.

What is actually happening?

We can not use a class instance so we have to add the locale switch logic inside the Vue.extend options: i.e. watch a vuex getter for current language and switch language through this.$i18n.locale.

[rikbrowning]

@kimuraz to get around this issue you must pass the i18n object into the constructor not the extend call.

const Constructor = Vue.extend(SmallComponent);
const vm = new Constructor({i18n}).$mount()

[jaredzhu1993]

@kazupon I’ve found the reason why Vue.extend cannot work, It’s because Vue.config.optionMergeStrategies.i18n use Vue.config.optionMergeStrategies.methods, which makes i18n instance loses his __proto__, so the options.i18n instanceof VueI18n becomes false. Will vue-i18n support Vue.extend({i18n})? If yes, I can fix this problem.