decorators execution does not come at the right time

[y-nk]

Bug Report

• I would like to work on a fix!
• Codesandbox

Input Code

function decorated(target, key, descriptor) {
  target.foo += "bar"

  const _ = target[key].bind(target)
  descriptor.value = () => {
    console.log("decorated")
    _()
  }
}

class Foo {
  foo = "foo"
}

class Bar extends Foo {
  @decorated bar() {
    console.log(this.foo)
  }
}

const foo = new Bar()
foo.bar()

Current behavior

Code stack executes as following:

1. Decorator is executed with an unknown/undesired instance of Foo without class properties set (not Bar, and target.foo === undefined)
2. Create a new Bar instance
3. Call constructor of Bar
4. Call constructor of Foo
5. Class properties are injected
6. foo.bar() is called with decorated version

Expected behavior

Code stack should execute as following:

1. Create a new Bar instance
2. Call constructor of Bar
3. Call constructor of Foo
4. Class properties are injected
5. Decorators are called
6. foo.bar() is called with decorated version

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Filename: babel.config.js

{
  "presets": [
    ["@babel/preset-env", {
      "targets": "last 2 versions"
    }]
  ],
  "plugins": [
    ["@babel/plugin-proposal-decorators", {
      "legacy": true
    }],
    ["@babel/plugin-proposal-class-properties", {
      "loose": true
    }]
  ]
}

Environment

• Babel version(s): @latest (v7.10.4)
• Node/npm version: not relevant (also fails in browser)
• OS: tested on macOS, but assumed cross-platform
• How you are using Babel: babel-loader with webpack (codesandbox uses parcel)

[babel-bot]

Hey @y-nk! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."