Reproducer: https://godbolt.org/z/c18xfdjYE The implementation partition unit is importable doesn't mean it must be imported. We can use it as a normal source file to implement entities in interfaces. This is better than using implementation units since now we can control the dependency in a much more finer grained manner. If some one wants to import it but didn't place it into CXX_MODULES FILES, it is user's fault. Note that now we can make it work by putting the implementation partition unit into CXX_MODULES FILES. But this is suboptimal since now it asks the users to pay for more time to write the BMI and more space to save the BMI.