Error: Cannot find module 'postcss-flexbugs-fixes' after upgrading to 9.3.4

[pjaws]

Bug report

Describe the bug

next build fails with the error in the title after updating to 9.3.4 with a custom PostCSS config following the current documentation

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Create a postcss.config.js with the contents specified here
2. Update to 9.3.4
3. No more builds!

Expected behavior

Documentation needs to be updated with the correct "base" PostCSS config for 9.3.4. I know you apparently removed postcss-flexbugs-fixes, but I have no idea what else might have changed.

[Timer]

You need to install these dependencies yourself, otherwise you're relying on an implicit dependency from Next.js. We might need to update the docs to make this more clear.

[timneutkens]

These aren't MY dependencies, they're yours. I wouldn't have them in this file if you didn't tell me to.

The moment you reference a dependency in your application configuration like postcss.config.js it's a dependency of your project.

Definitely can be mentioned in the docs that you have to install it, but you can't reference internal npm dependencies of other dependencies (like next.js) in a safe way as dependencies can change at any time.

An example would be (generic node.js example), you add fs-extra to your project and then import rimraf without adding it to your dependencies.

If you want to rely on Next.js configuring postcss you don't add postcss.config.js, otherwise it's up to you as a user to configure it and add the correct dependencies.

postcss-flexbug-fixes was not removed from Next.js for example.

[pjaws]

That's fair, but the documentation should also be updated when you change something like this. If a user right now goes and follows the advice in your docs, their application will break!

Again, the ideal solution would be to offer a way of extending or importing your base config.

[timneutkens]

Let's keep this open to improve the docs, mention installing the deps basically.

[pjaws]

I'd gladly update the docs if I knew what config you were using now.

[timneutkens]

It's the same config, only thing that has to be mentioned is that the user has to install the mentioned dependencies like postcss-flexbugs-fixes through npm install postcss-flexbugs-fixes etc

[timneutkens]

Fixed by #11966. Thank you @pjaws 💯

[balazsorban44]

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