Submodule semantics with macro guard

[Manman via cfe-commits]

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