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Re: Module search options

From: David Blaikie <dblaikie_at_[hidden]>
Date: Tue, 22 Feb 2022 17:48:18 -0800
On Mon, Feb 21, 2022 at 5:01 AM Boris Kolpackov <boris_at_[hidden]>

> 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 Including an outdated or wrong header may trigger a fatal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.

Received on 2022-02-23 01:48:30