Regression in Next.js 16.0.1: sassOptions.additionalData causes error when using --webpack flag, worked in 15.5.2

[GlebKodrik]

Link to the code that reproduces this issue

https://github.com/GlebKodrik/scss-nextjs/tree/master

To Reproduce

1. Use Next.js version 16.0.1 with the --webpack flag.
2. Add the following configuration to next.config.ts:

sassOptions: {
  additionalData: '@use "app/styles" as *;',
},

3. Start the development server using next dev or make a production build with next build.
4. An error occurs during the build process.

Can't find stylesheet to import.
  ╷
1 │ @use "app/styles" as *;

Current vs. Expected behavior

Expected Behavior: The configuration with sassOptions.additionalData should work identically to version 15.5.2, where it successfully injects the global SCSS module.

Current Behavior: In version 16.0.1, adding the sassOptions.additionalData line immediately causes a build error. This is a regression, as the same configuration works without any issues in version 15.5.2.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0
Binaries:
  Node: 20.17.0
  npm: 10.8.2
  Yarn: 1.22.22
  pnpm: 9.10.0
Relevant Packages:
  "next": "^16.0.1",
  "react": "19.1.0",
  "react-dom": "19.1.0"
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

CSS

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local)

Additional context

The issue is specific to the new version 16.0.1. Rolling back to 15.5.2 resolves the problem immediately.

The project uses the --webpack configuration flag.

The goal of the additionalData option is to globally import an SCSS module (app/styles) into every Sass file in the project.

[dstrizhakov]

Up! I have the same issue!

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[mischnic]

The difference is that sass-loader was upgraded from v15 to v16: #84387
The next config sassOptions are basically forwarded to that, so sass-loader must behave differently here: https://github.com/webpack/sass-loader/releases/tag/v16.0.0

[github-actions]

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