But we're not talking about header files, we're talking about modules. Modules are a new concept so how they work is up for definition. To say that if bar.cpp u

But we're not talking about header files, we're talking about modules. Modules are a new concept so how they work is up for definition. To say that if bar.cpp uses module foo that foo.bmi must already exist is not an unreasonable rule. modules work with the import statement (new) not the #include statement. They are not the same as include at all. In fact this is spelled out in the article in the first goal > The “importer” of a module cannot affect the content of the module being imported. The state of the compiler (preprocessor) in the importing source has no bearing on the processing of the imported code. In other words, the flags passed in when compiling bar.cpp have no effect on foo.bmi. foo.bmi is the result of the flags passed in when foo.cpp was compiled and those flags can only be gotten from the build system if foo.bmi does not exist.