Vue-next: v-model is not working in custom components

[fawmi]

Version

3.0.0-beta.20

Reproduction link

https://codesandbox.io/s/confident-noether-6i1q7?file=/package.json

Steps to reproduce

Create a new project with the latest Vue.js version.
Create a custom component and add a v-model attribute,
emit a value, the value is not updated in the parent component.

Lets say we have a child component

<template>
    <input :value="value" @input="emitUpdates">
</template>

<script>
    export default {
        props: ["value"],
        setup(props, { emit }) {
            function emitUpdates(event) {
                emit("input", event.target.value);
            }

            return {
                emitUpdates
            };
        }
    };
</script>

And a parent component

<template>
  <div>
    <Input v-model="state.someText" type="text"/>
    {{ state.someText }}
  </div>
</template>
<script>
import { reactive } from "vue";

export default {
  setup() {
    const state = reactive({
      someText: "Hallo"
    });

    return {
      state,
    };
  }
};
</script>

The value of state.someText should get updated if the input value is updated.

This is a working example in Vue2:
https://codesandbox.io/s/custom-input-component-v-model-23x1t

I don't know whether something is changed which is not documented or it is a bug.

What is expected?

The value in the parent component should be updated after emit("input", value) is called.

What is actually happening?

The value in the parent component is not updated after emit("input", value) is called.

[fawmi]

I don't know whether it is expected behavior or not, but it works, when specifying a model

v-model:value="state.someText"

[yyx990803]

Please read the RFC: https://github.com/vuejs/rfcs/blob/master/active-rfcs/0011-v-model-api-change.md