Using "loose" option in env preset results in invalid Set.concat() call

[joeporpeglia]

Bug Report

Current Behavior
Using a spread operator on a Set as described in the MDN docs while setting "loose": true for @babel/preset-env transpiles to the following:

var set = new Set([1, 2, 3, 4]);
set.concat().map(function (v) {
  return console.log(v);
});

Input Code

• REPL or Repo link if applicable:

const set = new Set([1,2,3,4]);
[...set].map(v => console.log(v));

Expected behavior/code
I would expect some sort of conversion from Set to Array before calling concat, similar to the approach with loose mode disabled.

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

{
  "presets": [
    ["@babel/env", {
      "targets": {
        "browsers": [
          ">2%",
          "last 1 Edge version",
          "last 2 Safari version",
          "last 1 Firefox version"
        ]
      },
      "loose": true
    }]
  ]
}

Environment

• Babel version(s): v7.0.0-beta.47
• Node/npm version: Node 8 / npm 5
• OS:OSX 10.12.6
• Monorepo: no
• How you are using Babel: cli and loader

Possible Solution
With loose mode enabled it seems like the spread transform assumes all sets are transpiled to arrays. I don't think that's always the case though, depending on the browser's being targeted. I don't have a solution to propose, but hopefully these details can help.

Additional context/Screenshots
none

[babel-bot]

Hey @joeporpeglia! 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]

That is the expected behavior of loose, since the goal of loose is to simplify the assumptions that Babel makes about spec compliance. I would not recommend loose unless you've looked through all the plugins you're using to see if the limitations it imposes are actually something you'd want.

https://www.npmjs.com/package/@babel/plugin-transform-spread#options

In loose mode, all iterables are assumed to be arrays.

[joeporpeglia]

Ah I should've checked the docs for the plugins included by env. Thanks for the help @loganfsmyth!