Current `-Mmodules` output is based on [P1602R0](wg21.link/p1602r0), which speaks about a set of Makefile rules that can handle modules, with the help of module mappers and a modified GNU Make.
The proposal came out in 2019, and the output of those rules was implemented at GCC in 2020. However, so far we still don't have a new release of GNU Make which implements P1602R0. What's more, the rules described in P1602R0 are not ideal. It sets up phony prerequisites for real-file targets, causing guaranteed rebuilds. It is also unable to handle dependencies among module interfaces - that is to say, if module A imports and exports B, then the interface of module A depends on that of module B, so its CMI should be rebuilt if the interface of B changes. I tried a few approaches to fix the current implementation, but in vain. It is possible, however, to have another set of Makefile rules generated, which solves all the problems, and doesn't need a new GNU Make. I've posted it in reddit. See [here](https://www.reddit.com/r/cpp/comments/1izg2cc/make_me_a_module_now/). To briefly summarize the idea: > If an object target is built from a module interface unit, the rules generated > are: > > ```Makefile > target.o: source.cc regular_prereqs header_unit_prereqs| header_unit_prereqs \ > module_prereqs > source_cmi.gcm: source.cc regular_prereqs header_unit_prereqs \ > module_prereqs| target.o > ``` > If an object target is not, the rule generated is: > > ```Makefile > target.o: source_files regular_prereqs \ > header_unit_prereqs| header_unit_prereqs module_prereqs > ``` > > The `header_unit_prereqs` and `module_prereqs` are paths to the corresponding > CMI files. The patched GCC can be found [here](https://github.com/vspefs/gcc/tree/module-makefile-gen-fix). An example project can be found [here](https://github.com/vspefs/makefile-cxx-module). Best regards, vspefs
