[boris]

> Header units are supposed to partially address this but right now they are not supported in any build systems properly (except perhaps msbuild?).

They are supported in build2 when used with GCC (via the module mapper mechanism it offers). In fact, I would be surprised if they were supported by msbuild, provided by "properly" we mean without having to manually specify dependencies involving header units and without imposing non-standard limitations (like inability to use macros exported by header units to conditionally import other header units).

[pjmlp]

VC++ has support header units for quite some time, in fact I had to revert back to global module fragments, because CMake/clang still don't have a plan for how to support header units, and I wanted to have my demo code work in more than just VC++.