By the way, what's stop us from having compiler options like `g++ -Rgcm.cache -Rsomewhere/else/gcm.cache` to specify CMI repo path, like `-I` for include paths? It could be useful for projects with complex folder structure, as build tools like Make sometimes change current working directory, and so we need to locate CMIs in different folders.
The mapping between module interface unit, module name, and expected CMI filename is still performed by the module mapper. But now when looking up a CMI, it goes to each repo in the list, in order, until it finds a CMI that matches and returns its full path. When producing a CMI, the CMI file is dumped to the first repo. Ideally, all invocations concerning modules should have `-Rgcm.cache` as the first CMI repo. This way, all CMI producing calls remain deterministic, and behave same as before. To talk about its implementation, since GCC has already written a customed resolver for the built-in module mapper, all we need to do is adding a driver option, editing the existing c++tools/resolver.cc, and implementing the option logic. This could make Make-based build systems really work. The Makefile rules proposed in this RFC make sure CMIs are built before used, and this `-R` flag offers big-project-ready module lookup mechanics, if we just ignore the multiple CMIs problem for now :( This option alone, I believe, could also offer some convenience to the general usage of modules. I have yet come up with a reference implementation! It should be quick work, though. Please share your ideas and suggestions. Best regards, vspefs
