Can't use exportLocalsOnly in css-loader

[Istanful]

Bug report

When I configure mini-css-extract-plugin in conjunction with css-loader and exportLocalsOnly I encounter an error.

Reproducible repo: https://github.com/Istanful/export-locals-only-mini-css-error

Actual Behavior

When I configure mini-css-extract-plugin in conjunction with css-loader and exportLocalsOnly I encounter an error.

Expected Behavior

It should ignore that no default export exists and compile anyways.

How Do We Reproduce?

I create a webpack.config.js file. This is the relevant configuration.

        {
          test: /\.css$/i,
          use: [
            {
              loader: MiniCssExtractPlugin.loader,
            },
            {
              loader: "css-loader",
              options: {
                esModule: true,
                modules: {
                  namedExport: true,
                  exportLocalsConvention: "camelCaseOnly",
                  localIdentName: "[path][name]__[local]--[hash:base64:5]",
                },
              },
            },
          ],
        },

When I then run webpack serve I get the following error:

TypeError: Cannot use 'in' operator to search for 'locals' in undefined

It happens here as it tries to check if locals is in the default export.

I wish to not have a default export as I want to only use CSS module locals.

Please paste the results of npx webpack-cli info here, and mention other relevant information

  System:
    OS: macOS 10.15.7
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
    Memory: 561.29 MB / 16.00 GB
  Binaries:
    Node: 12.18.3 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.6 - /usr/local/bin/npm
  Browsers:
    Chrome: 94.0.4606.71
    Safari: 14.1.2
  Packages:
    extract-text-webpack-plugin: ^3.0.2 => 3.0.2
    webpack: ^5.54.0 => 5.54.0
    webpack-cli: ^4.8.0 => 4.8.0
    webpack-dev-server: ^4.3.0 => 4.3.0

[alexander-akait]

Please provide reproducible test repo

[Istanful]

@alexander-akait Sure thing. Here you go: https://github.com/Istanful/export-locals-only-mini-css-error

[alexander-akait]

Fixed - #844, note - you should avoid using mini-css-extract-plugin with exportLocalsConvention, it just doesn't make sense because you don't have CSS inside js file, so nothing to extract, but you will decrease build time on checks inside plugin

[Istanful]

Thanks! You're right, that makes sense. I did not quite think that through. :P

I'm hoping to only use scoped classes and forbidding global selectors such as body. My understanding was that exportLocalsOnly did this and mini-css-extract-plugin would split the CSS appropriately.