It is possible that proguard merges base module and feature module classes or inlines base module methods into feature module classes. Currently, merged classes are moved into the base module. However, this may cause DEX verification problems when the merged class references other feature module classes that were not moved into the base module. It's unclear how likely that is to happen in practice. For now, just fail the build if such a merging did happen so that the error condition is clearer.