Redefined specialization in type_traits.h using MSVC (Protobuf 3.1.0)

[mrry]

This commit added explicit specializations for is_integral<__int64> and is_integral<unsigned __int64 when compiling with Visual C++: 3c88e1b

It seems that by default HAVE_LONG_LONG is not defined on this platform, so these specializations do not collide with the specializations for is_integral<long long> and is_integral<unsigned long long> immediately above them.

However, other projects use HAVE_LONG_LONG on Windows, which leads to an error about the specialization already being defined. In particular, when I include <Python.h> from Anaconda for Windows before a header that includes some protobuf headers, I get the following compilation error:

[...]google/protobuf/stubs/type_traits.h(151): error C2766: explicit specialization; 'google::protobuf::internal::is_integral<__int64>' has already been defined
[...]google/protobuf/stubs/type_traits.h(143): note: see previous definition of 'is_integral<__int64>'
[...]google/protobuf/stubs/type_traits.h(152): error C2766: explicit specialization; 'google::protobuf::internal::is_integral<unsigned __int64>' has already been defined
[...]google/protobuf/stubs/type_traits.h(144): note: see previous definition of 'is_integral<unsigned __int64>'

Here's a simple repro, test.cc:

//#include <Python.h>  // The following #define has the same effect.
#define HAVE_LONG_LONG
#include "google/protobuf/stubs/type_traits.h"

int main(int argc, char** argv) {
  return 0;
}

Reproduced using the following command line with Visual Studio 2015 Update 1:

C:\temp> cl.exe test.cc /I<path\to\protobuf\src\>

Could the conditional compilation be refactored to avoid this problem? @xfxyjwf, since you made the commit, any ideas?

[xfxyjwf]

When reading https://msdn.microsoft.com/en-us/library/29dh1w7z.aspx, it says:

The __int64 type has no ANSI equivalent.

That made me think __int64 is different from long long on MSVC. Apparently it's either my reading is wrong or the MSVC doc is lying.

I guess we can change it to:

#if defined(HAVE_LONG_LONG) || defined(_MSC_VER)
template<> struct is_integral<long long> : true_type { };
template<> struct is_integral<unsigned long long> : true_type { };
#endif

Or maybe just drop the #ifdef/#endif entirely as compilers that don't support "long long" are probably too old to compile protobuf code any way.

Can you help send a pull request to fix that?

[mrry]

Thanks for the quick response! Yes, I'll send a quick Pull Request with your first suggested change (after testing it with the code I'm building).