On Mon, Feb 21, 2022 at 5:01 AM Boris Kolpackov <boris_at_[hidden]>
wrote:

> David Blaikie via SG15 <sg15_at_[hidden]> writes:
>
> > (I hope this isn't too derailing... )
> >
> > Seems like we have, broadly:
> >
> > 1) Lookup relative to/related to the include/module source search path
>
> I would strongly advocate against adding any sort of search paths
> for modules.
>
> I am not sure what other's experience is with header search paths
> (-I), but ours in build2 strongly suggests that it's probably the
> most broken part of the C/C++ compilation model, especially when
> it comes to auto-generated headers. Specifically:
>
> 1. Compilers have built-in header search paths which means build
> systems have to figure out what they are, which is invariably
> done in a hackish, compiler-specific manner.
>
> 2. The search paths are sensitive to order which makes using
> independently-developed libraries in the same build brittle,
> with wrong headers being found in unexpected locations.
>
> The practice of including headers with a library-specific
> directory prefix (e.g., <libfoo/utility.hpp>) is unfortunately
> nowhere near universal (even this committee members, who must
> know better, do not always follow it).
>
> 3. If an auto-generated header does not exist, the compiler may
> find and include an identically-named but unrelated header that
> is found in one of the further -I directories.
>
> Including an outdated or wrong header may trigger a fatal
> preprocessor error before the build system has a chance to
> correct the situation (e.g., re-generate the header in the
> expected location).
>
> See P1842[1] for further detail on this point and what could be
> done about it.
>
> Please, let's not repeat the same mistake again.
>
> [1] https://wg21.link/P1842


Ah. Have you got some thoughts about how this does or doesn't relate to
Daniel's https://isocpp.org/files/papers/P2473R0.pdf & related discussions?
While the compiler might not search these paths - the build system may
benefit from/'need' (for various values of need) some generic system to
find module source, and related information on how to build those modules,
for modules defined outside the immediate project.