Extend options should not be confused with the super options

[z-ZYS-s]

A simple case:

const Base = Vue.extend({
  computed: {
    a () { return 'aa' }
  }
})
const Sub = Base.extend()
Sub.options.computed.b = () => 'bb'

console.log(Base.options.computed.b()) // 'bb'
console.log(Base.options.computed === Sub.options.computed) // true

The reason has been found, in line 181, vue/src/core/util/options.js

strats.props =
strats.methods =
strats.computed = function (parentVal: ?Object, childVal: ?Object): ?Object {
  if (!childVal) return parentVal

It just return the parent object. Maybe it should be replaced by code like this

strats.props =
strats.methods =
strats.computed = function (parentVal: ?Object, childVal: ?Object): ?Object {
  if (!childVal) return extend({}, parentVal)

There is a same problem for strats.watch in line 158.
There is no need to change strats.data in line 69 because the type is function.

I'd like to submit a PR but I'm not familiar with the code, like that props could be a Array so more things should be done after `extend({}, parentVal)...

Hope someone can fix this bug, Thanks!

---

Actually, it is not commonly used to set Sub.options.computed directly, but I met this bug when using vue-loader with css module. For example:

base.vue

<template>
<div :class="$style.base">base<div>
</template>
<script>
import Vue from 'vue';

export default Vue.extend({
  name: 'my-base',
});
</script>
<style module>
.base {
  color: red;
}
</style>

sub.vue

<template>
<div :class="$style.sub">sub<div>
</template>
<script>
import Base from './base.vue';

export default Vue.extend({
  name: 'my-sub',
});
</script>
<style module>
.sub {
  color: blue;
}
</style>

And finally, you will find that the computed property $style of <my-base> has been replaced by the one of <my-sub>. They share the same $style in fact.

The following lines using options directly are in vue-loader:

• __vue_options__ = __vue_options__.options in line 254, vue-loader/lib/loader.js
• __vue_options__.computed[key] = function () { return module } in line 285

And with the mentioned bug above in vue cause the final problem.