Submodule semantics with macro guard

[Manman via cfe-commits]

> On Sep 19, 2016, at 5:55 PM, Richard Smith via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> 
> Your c.h is not correct. It would introduce a definition of c in every file where it's included, so it's not a modular header.
Hi Richard,

What do you mean by c.h is not correct? It is guarded by a macro, so if we are not using modules, it will work.

Also I thought the definition of a non-modular header is that it is not mapped to any module. Is that my misunderstanding? Is there a formal definition for modular header?

Thanks,
Manman
> 
>> On 19 Sep 2016 5:21 pm, "Manman via cfe-commits" <cfe-commits at lists.llvm.org> wrote:
>> 
>> Hi Richard & Ben,
>> 
>> Given a simple testing case, where we have two submodules X.A (A.h) and X.B (B.h, it includes C.h, and C.h is guarded with a macro), when we import X.A and then textually include a header C.h, we get redefinition error. This is because the macro guard is owned by module X.B that is not yet visible. I wonder what the fix is if this is considered a compiler issue. We can:
>> 1> do not emit the redefinition error if the earlier definition is not visible, or
>> 2> emit diagnostics to suggest user to import the whole module, when redefinition happens because the module includes a non-modular header and the non-modular header is also textually included, or
>> 3> other suggestions?
>> 
>> Note that if we textually include C.h, then import X.A, there are no compiler errors.
>> 
>> The testing case here:
>> clang -cc1 repro.c -fsyntax-only -I Inputs4/ -fmodules -fimplicit-module-maps -fmodules-cache-path=tmp
>> cat repro.c
>> #include "A.h" //modular header
>> #include "C.h" //non-modular header
>> 
>> cat Inputs4/module.map
>> module X {
>>   module A {
>>     header "A.h"
>>     export *
>>   }
>>   module B {
>>     header "B.h"
>>     export *
>>   }
>>   export *
>> }
>> cat Inputs4/A.h
>> #ifndef __A_h__
>> #define __A_h__
>> #endif
>> cat Inputs4/B.h
>> #ifndef __B_h__
>> #define __B_h__
>> #include "C.h"
>> #endif
>> cat Inputs4/C.h
>> #ifndef __C_h__
>> #define __C_h__
>> int c = 1;
>> const int d = 2;
>> #endif
>> 
>> Thanks,
>> Manman
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