Async function hoisting is overly aggressive

[houli]

Bug Report

The transformation of async function x to var x = function () { var _ref = _asyncToGenerator(... means that x must be hoisted. This is to avoid issues where x is referred to before it is defined. The problem is that the hoisting is overly aggressive and async functions appear before other statements in the code such as requires and imports. This is immediately noticeable if you try to write an async function in a module that sets up babel-polyfill/regenerator-runtime as a global using a side-effectful require/import. The result is broken code that fails as regeneratorRuntime will be undefined.

Input Code

Example using import
Example using require

Babel Configuration (.babelrc, package.json, cli command)

{
  "presets": ["env"]
}

Expected Behavior

The order of statements is preserved in a way that async functions are hoisted only as high as they need to be.

Current Behavior

Async functions are hoisted to the top of the module including to the point where they break the semantics of the import statement by appearing before it.

Your Environment

software	version(s)
Babel	6.26.0
node	8.4.0
npm	5.3.0
Operating System	macOS

[babel-bot]

Hey @houli! We really appreciate you taking the time to report an issue. The collaborators
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so it's possible this won't be addressed swiftly.

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[loganfsmyth]

Currently this is the suggested approach: #4487 (comment)

We don't really have any way of knowing where to put the function. We could technically special-case babel-polyfill, but you could just as easily do import "./my-polyfills"; with babel-polyfill in another file and it would still break.

The best we could do is leave it as a function declaration and just rewrite it to remove the async instead, which would still break if you managed to call the function before the polyfill had executed. We may well do that at some point, but it hasn't happened yet because the recommendation above works for all cases and is easy to do.

[loganfsmyth]

This should be fixed in 7.x