Upgrade to v7 stops module concatentation from working

[jacobwindsor]

v7 Regression

First check out: https://new.babeljs.io/docs/en/next/v7-migration.html
Also a partial upgrade tool: https://github.com/babel/babel-upgrade

Potential Commit/PR that introduced the regression
I'm unsure if this is related to a change in babel or babel loader. I would guess babel but have been unable to find a PR related to it. However, PR #660 in babel-loader is related.

Describe the regression
When upgrading to babel 7, webpack's module concatenation does not behave correctly. Everything builds okay but results in a runtime error:

Disabling optimization.concatenateModules in the webpack production config fixes this. optimization.concatenateModules is disabled by default in the development mode so this issue only arises in production.

As expected from previously mentioned babel-loader PR, setting modules to commonjs in the babel config also fixes the issue.

Both of the above fixes just force module concatenation to be disabled.

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

{
    "presets": [["@babel/env", {
        "targets": {
            "browsers": ["last 2 versions", "ie >= 9"]
        }
        , "useBuiltIns": "entry"
    }]]
    , "plugins": ["@babel/plugin-syntax-dynamic-import"]
}

Expected behavior/code
Webpack's module concatenation should behave as expected with Babel 7.

Environment

• Babel version(s): 7.0.0
• Node/npm version: 8.9.4
• OS: windows 10
• babel-loader: 8.0.4
• How you are using Babel: loader

[babel-bot]

Hey @jacobwindsor! 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
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[jacobwindsor]

Module concatenation would have been disabled before I upgraded to babel 7 too. I hadn't disabled module transformation in the babel config.

So, this might not be a babel issue but I'm unable to confirm whether it's a webpack issue. I'll leave it here in the hope that someone might have an idea.

[loganfsmyth]

Not sure specifically, but maybe this is related to webpack/webpack#4039 (comment)? Are you compiling any CommonJS files in your project? If not, it's possible you're doing something else that Webpack doesn't expect. I'm not sure, I'd have to see the actual code that throws the error.

[jacobwindsor]

Thanks for the tip @loganfsmyth

Unfortunately, I can't share the code. It doesn't seem like our project is subject to any of those two conditions. We are only transpiling our source code and not node_modules and are using harmony imports/exports everywhere. The only thing I can think is that we have one file that contains many harmony imports and one require statement. The require statement is needed as it is importing a commonjs module.

I have tried adding sourceType: unambigous but it does not help.

[danez]

Can you try adding modules: false to the preset-env config? In theory babel should autodetect that webpack supports esmodules and set this configuration, but if this fixes the problem, then we definitely know it is a problem in babel.

{
    "presets": [["@babel/env", {
        "targets": {
            "browsers": ["last 2 versions", "ie >= 9"]
        }
        , "modules": false
        , "useBuiltIns": "entry"
    }]]
    , "plugins": ["@babel/plugin-syntax-dynamic-import"]
}

[jacobwindsor]

@danez That does not fix the issue. Still exactly the same problem. Why does this mean it;s a babel issue?

[nicolo-ribaudo]

If not transforming modules with Babel doesn't solve the issue, then it is a bug with webpack.

[jacobwindsor]

Closed and moving to webpack repo