clang-cl treats different extern "C" declarations as redeclarations

[arrowd]

Bugzilla Link	29142
Version	3.9
OS	Windows NT
CC	@DougGregor,@zygoloid,@rnk

Extended Description

The code

namespace acommon {

struct ErrorInfo;

extern "C" __declspec(dllexport)  const ErrorInfo * const aerror_other;

}

extern "C" {
struct AspellErrorInfo {

  const struct AspellErrorInfo * isa;

  const char * mesg;

  unsigned int num_parms;

  const char * parms[3];

};


typedef struct AspellErrorInfo AspellErrorInfo;
extern const struct AspellErrorInfo * const aerror_other;
}

int main()
{
return 0;
}

compiles with cl.exe, but fails with clang-cl.exe:

tst.cpp(25,45):  error: redeclaration of 'aerror_other' with a different type: 'const struct AspellErrorInfo *const' vs 'const acommon::ErrorInfo *const'
extern const struct AspellErrorInfo * const aerror_other;
                                            ^
tst.cpp(6,59):  note: previous declaration is here
extern "C" __declspec(dllexport)  const ErrorInfo * const aerror_other;
                                                          ^

[rnk]

MSVC's model doesn't make sense. They accept this code and generate an object file with two bar symbols:

namespace foo { extern "C" int bar = 1; }
extern "C" int bar = 2;

Your code looks more like this:

namespace foo { extern "C" int bar = 1; }
int bar = 2;

... which doesn't produce an invalid object with MSVC, so it's not obviously invalid.

I suspect that that code is ill-formed, though. If you can make the case that there is a large body of existing code with this issue, we might find a way to accept this pattern. Otherwise my inclination is to say that this is working as intended.

[arrowd]

MSVC's model doesn't make sense. They accept this code and generate an
object file with two "bar" symbols:

namespace foo { extern "C" int bar = 1; }
extern "C" int bar = 2;

Your code looks more like this:

namespace foo { extern "C" int bar = 1; }
int bar = 2;

... which doesn't produce an invalid object with MSVC, so it's not obviously
invalid.

I suspect that that code is ill-formed, though. If you can make the case
that there is a large body of existing code with this issue, we might find a
way to accept this pattern. Otherwise my inclination is to say that this is
working as intended.

Well, this is what i get when i try to compile aspell (as you might guessed) on Windows. I can't judge which compiler is right, so feel free to close this bug if you think that clang does right thing.

[zygoloid]

Because both declarations of aerror_other are declared in extern "C" contexts, they are redeclarations of the same variable; see [dcl.link]/6:

Two declarations for a variable with C language linkage with the same name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same variable.

So this is definitely ill-formed, because the same variable has been declared with two different types, just as it would be if both declarations were in the same namespace. (Most of the other extern "C" rules exist to allow for various name mangling patterns, and we could reasonably relax those for MSVC in cases where the relevant mangling collisions don't occur, but accepting this case seems much less reasonable.)

Note that this isn't only a problem on Windows; aspell's headers are apparently just broken when included from C++:

http://stackoverflow.com/questions/25395685/aspell-wont-build-on-os-x-10-9-mavericks

[AaronBallman]

Duplicate of #95681

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Gleb Popov (arrowd)

| | | | --- | --- | | Bugzilla Link | [29142](https://llvm.org/bz29142) | | Version | 3.9 | | OS | Windows NT | | CC | @DougGregor,@zygoloid,@rnk |

Extended Description

The code

namespace acommon {

struct ErrorInfo;

extern "C" __declspec(dllexport)  const ErrorInfo * const aerror_other;

}

extern "C" {
struct AspellErrorInfo {

  const struct AspellErrorInfo * isa;

  const char * mesg;

  unsigned int num_parms;

  const char * parms[3];

};


typedef struct AspellErrorInfo AspellErrorInfo;
extern const struct AspellErrorInfo * const aerror_other;
}

int main()
{
return 0;
}

compiles with cl.exe, but fails with clang-cl.exe:

tst.cpp(25,45):  error: redeclaration of 'aerror_other' with a different type: 'const struct AspellErrorInfo *const' vs 'const acommon::ErrorInfo *const'
extern const struct AspellErrorInfo * const aerror_other;
                                            ^
tst.cpp(6,59):  note: previous declaration is here
extern "C" __declspec(dllexport)  const ErrorInfo * const aerror_other;
                                                          ^