Turbopack: :global CSS selector fails with "Ambiguous CSS module class not supported"

[graulund]

Link to the code that reproduces this issue

https://github.com/graulund/next-turbopack-global-css-issue

To Reproduce

1. Start the app in development using Webpack (npm run dev)
2. Observe the page loads without error
3. Start the app in development using Turbopack (npm run dev-turbo)
4. Observe a build error on line 7 of styles.module.css ("Ambiguous CSS module class not supported")

Current vs. Expected behavior

I would expect the same CSS module code to work regardless of whether Webpack or Turbopack is used. That is, I would expect both :global(...) and :global ... to work. Right now, only the first one works in Turbopack. Both seem to be valid expressions in CSS modules (see https://css-modules.github.io/webpack-demo/, section "Global Selectors").

Alternatively, I would expect this relatively cryptic error message to be described in detail somewhere. I found nothing on Google searching for this error message ("Ambiguous CSS module class not supported").

Instead of the same CSS module code working regardless of whether Webpack or Turbopack is used, I get this error on Turbopack:

Parsing css source code failed
   5 |
   6 | /* Works with Webpack, fails on Turbopack */
>  7 | .container :global .test {
     |              ^
   8 | 	color: red;
   9 | }
  10 |

Ambiguous CSS module class not supported at [project]/app/styles.module.css:6:13

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:23 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6020
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 23.7.0
  npm: 11.2.0
  Yarn: N/A
  pnpm: 10.6.5
Relevant Packages:
  next: 15.3.0-canary.23 // Latest available version is detected (15.3.0-canary.23).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
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)

Additional context

Perhaps vaguely related to #74309, although that PR seems to be addressing a different issue (erroneous filename).