Bug when mixing async/await and spreading on function's arguments

Bug Report

• I would like to work on a fix!

Why not if I can have some hints on where to look for 🤔

Current Behavior A clear and concise description of the behavior.

It seems that the code produced when transpiling (await p).print(...args) using @babel/preset-env with its default configuration is not doing the same as the original code. The print function is called with an undefined context while I expected it to be called with the context of the result of await p.

By the way a quick fix (in my case) was to extract await p into a variable first and call print on this variable right after - see the linked repo. The bug only occur when I have both await and ...args on the same statement.

Input Code

• REPL or Repo link if applicable: https://github.com/dubzzz/babel-async-bug

async function code(p, ...args) {
  // Original code: Succeeds
  // Transpiled code: Fails with "TypeError: Cannot read property 'x' of undefined"
  console.log((await p).print(...args));
}

// Example
const instance = {
  x: 0,
  y: 1,
  print() {
    return `${this.x} ${this.y}`;
  }
};
const promiseInstance = Promise.resolve(instance);
code(promiseInstance);

Expected behavior/code A clear and concise description of what you expected to happen (or code).

I expect the transpile version of the code to behave the same way as the original source code.

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

• Filename: .babelrc

{
  "plugins": [
    [
      "@babel/plugin-transform-runtime",
      {
        "regenerator": true
      }
    ]
  ],
  "presets": ["@babel/preset-env"]
}

Environment

  System:
    OS: Windows 10 10.0.19041
  Binaries:
    Node: 12.11.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.19.1 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.11.3 - C:\Program Files\nodejs\npm.CMD

• Babel version(s):
  • “@babel/cli”: “7.8.4”,
  • “@babel/core”: “7.8.7”,
  • “@babel/plugin-transform-runtime”: “7.8.3”,
  • “@babel/preset-env”: “7.8.7”
• How you are using Babel: cli

Possible Solution

Additional context/Screenshots Add any other context about the problem here. If applicable, add screenshots to help explain.

[nicolo-ribaudo]

Thanks for the repository! It makes it way easier to debug this bug.

I managed to make a simpler reproduction:

function* code() {
  var _ref;
  console.log((_ref = yield).print.call(_ref));
}

const instance = {
  x: 0,
  y: 1,

  print() {
    return this === instance;
  }

};
const it = code();

it.next();
it.next(instance);

{
  "sourceType": "script",
  "plugins": [
    [
      "@babel/plugin-transform-runtime",
      {
        "regenerator": true
      }
    ],
    "@babel/transform-regenerator"
  ]
}

PS. I noticed that you are using .babelrc. It works, but probably what you want is babel.config.js/babel.config.json!

[nicolo-ribaudo]

Fixed by https://github.com/facebook/regenerator/pull/386