Ben Boeckel <ben.boec...@kitware.com> writes: > 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.
Well, if scanning cannot require a pre-existing BMI but a pre-existing BMI is required to get accurate dependency information, then something has to give. You hint at a potential solution in your subsequent email: > Can't it just read the header as if it wasn't imported? AFAIU, that's > what GCC did in Jan 2019. I understand that CPP state is probably not > easy, but something to consider. The problem with this approach is that a header import and a header include have subtly different semantics around macros. In particular, the header import does not "see" macros defined by the importer while the header include does. Here is an example: // file: header-unit.hpp // #ifdef BAR #define FOO #endif // file: importer.cpp // #define BAR import "header-unit.hpp"; // Should not "see" BAR. //#include "header-unit.hpp" // Should "see" BAR. #ifdef FOO import "header-unit2.hpp"; #endif In this example, if you treat import of header-unit.hpp as include, you will get incorrect dependency information. So to make this work correctly we will need to re-create the macro isolation semantics of import for include. Even if we manage to do this, there are some implications I am not sure we will like: the isolated macros will contain inclusion guards, which means we will keep re-scanning the same files potentially many many time. Here is an example, assume each header-unitN.hpp includes or imports <functional>: // file: importer.cpp // import <functional>; // Defined _GLIBCXX_FUNCTIONAL include import "header-unit1.hpp"; // Ignores _GLIBCXX_FUNCTIONAL import "header-unit2.hpp"; // Ditto. import "header-unit3.hpp"; // Ditto. import "header-unit4.hpp"; // Ditto.
