For the complete context: symfony-cmf/routing-bundle#370

In short, there are classes that should not be used in some cases (for instance, the PHPCR document referenced in the issue if PHPCR is not installed and disabled). This class definition relies on some PHPCR interfaces, which do not exists.

As the cache warmer tries to load all metadata, this results in errors. This can be fixed by dynamically registering the validation metadata in a compiler pass based on the configuration.

However, it still is a BC break in the code. I just wanted to notify you about this, not sure what can be done to avoid this (checking all interfaces first seems lots of overhead). At least, the UPGRADE-3.2.md file should mention this.