Decorated async functions with retainLines is line-breaking after async

[andrewgies17]

Bug Report

Current behavior
Babel will generate invalid output from valid input when the following conditions hold:

• An async function is decorated
• Decorators are transformed using @babel/plugin-proposal-decorators in legacy (stage 1) mode.
• @babel/preset-env is used targeting a version of Node that does not require regenerator-runtime (i.e. 12)
• retainLines is set to true in the babel config

With this combination, the generated code will contain async and decorateMe() (in the below example), on two different lines. This does not make a valid async function declaration, and so node will complain about the presence of await in a non-async function.

Small Reproduction Repo: https://gitlab.com/andrew.gies.axiom/babel-repro

Input Code

const TestDecorator = (target, _property, descriptor) => {
  console.log("Decorated!");
};

class Test {
  @TestDecorator
  async decorateMe() {
    const text = await Promise.resolve('pinky swear');
    console.log(text);
  }
}

Expected behavior
This code will generate valid output that can be run by Node 12.

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

• Filename: .babelrc

{
  "presets": [
      ["@babel/env", {"targets": {"node": "12"}}]
    ],
  "retainLines": true,
  "plugins": [
    ["@babel/proposal-decorators", {"legacy": true}]
  ]
}

Environment

  System:
    OS: macOS Mojave 10.14.6
  Binaries:
    Node: 12.18.2 - ~/.nvm/versions/node/v12.18.2/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.6 - ~/.nvm/versions/node/v12.18.2/bin/npm
  npmPackages:
    @babel/cli: ^7.10.5 => 7.10.5 
    @babel/core: ^7.10.5 => 7.10.5 
    @babel/plugin-proposal-decorators: ^7.10.5 => 7.10.5 
    @babel/preset-env: ^7.10.4 => 7.10.4 

• Babel version(s): 7.10.5 & main (2bf38fb)
• Node/npm version: Node 12.18, npm 6.14
• OS: macOS Mojave 10.14
• Monorepo: no
• How you are using Babel: cli

Possible Solution
A workaround is removing any of the four necessary conditions listed above, the easiest of which is to ditch retainLines. This is currently what I'm doing for my project - we don't actually need retainLines, it's a vestige of an older time.

Additional Context
I did some digging and found the following historical info related to this problem:

• A related report (from Sep. 2018) can be found in #8650
• #8650 was closed in favor of a related issue: #7275
• #7275 was marked closed as a result of PR #8868 in Oct 2018
• A very similar (but not quite the same) issue just surfaced about a week ago: #11830
• #11830 was fixed by a recent PR: #11836
• This PR hasn't yet landed in a release. However, I can still reproduce this issue on main (by cloning the babel
  repo and yarn linking the latest contents of main, including the merged PR #11836, into this test repo.)

[babel-bot]

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