Polymorphic hasMany relathionship with ES6 classes

Questions
1

Let’s create an app with 3 models, User, Post and SpecialPost. They are all defined with the ES6 class/extends syntax thanks to ember-decorators.

// models/user.js
import DS from 'ember-data';
import { hasMany } from '@ember-decorators/data';
const { Model } = DS;

export default class UserModel extends Model {
  @hasMany() posts;
}

// models/post.js
import DS from 'ember-data';
const { Model } = DS;

export default class PostModel extends Model {}

// models/special-post.js
import Post from './post';

export default class SpecialPostModel extends Post {}

When trying to assign a special-post to a user, I get an error:

    const post = this.store.createRecord('special-post');
    return this.store.createRecord('user', {posts: [post]});
// =>  Assertion Failed: You cannot add a record of modelClass 'special-post' to the 'user.posts' relationship (only 'post' allowed)

Since SpecialPost is a subclass of Post, I thought this would automatically work…

One workaround I’ve found is to declare SpecialPost this way:

// models/special-post.js
import Post from './post';

// note the Post.extend() part
export default class SpecialPostModel extends Post.extend() {}

Unfortunately, this works under Ember 3.1 but does not under 3.2 (see Ember 3.2 + ember-decorators subclassing bug)

Here is a repo reproducing the bug:

How can I assign a special-post to a user only using ES6 class syntax without the extends Post.extend() hack?