[modules] Preprocessing requires compiled header unit modules

[Boris Kolpackov]

Ben Boeckel <ben.boeckel@kitware.com> writes:

> However, for header unit modules, it runs into a problem that imported
> header units are required to be compiled and available in the mapper
> while scanning for dependencies.
> 
> Example code:
> 
> ```c++ # use-header.cpp
> module;
> 
> import "header-unit.hpp";
> 
> int main(int argc, char* argv[]) {
>     return good;
> }
> ```
>
> There used to be no need to do this back prior to the modules landing in
> `master`, but I can see this being an oversight in the meantime.

I don't think it is. A header unit (unlike a named module) may export
macros which could affect further dependencies. Consider:

import "header-unit.hpp"; // May or may not export macro FOO.

#ifdef FOO
import "header-unit2.hpp"
#endif