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Date: Thu, 21 Apr 2022 08:05:33 -0400
From: Ben Boeckel <ben.boeckel@kitware.com>
To: Boris Kolpackov <boris@codesynthesis.com>
Cc: gcc@gcc.gnu.org
Subject: Re: [modules] Preprocessing requires compiled header unit modules
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On Thu, Apr 21, 2022 at 06:05:52 +0200, Boris Kolpackov wrote:
> 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

I agree that the header needs to be *found*, but scanning cannot require
a pre-existing BMI for that header. A new mode likely needs to be laid
down to get the information necessary (instead of just piggy-backing on
`-E` behavior to get what I want).

--Ben