Clang fails to build due to operator<< ambiguities

[davidchappelle]

I am trying to build latest msgpack and found that simple custom adaptors are not compiling using clang however they do compile using gcc. In particular I am seeing compilation errors such as these:

main.cpp:53:16: error: use of overloaded operator '<<' is ambiguous (with operand types 'msgpack::object::with_zone' and 'const std::string' (aka 'const basic_string<char>'))
        object << f.name();
        ~~~~~~ ^  ~~~~~~~~
/home/dchappelle/environment/staging/include/msgpack/v1/adaptor/adaptor_base.hpp:71:6: note: candidate function [with T = std::basic_string<char>]
void operator<< (msgpack::object::with_zone& o, T const& v) {
     ^
/home/dchappelle/environment/staging/include/msgpack/v2/adaptor/adaptor_base.hpp:66:6: note: candidate function [with T = std::basic_string<char>]
void operator<< (msgpack::object::with_zone& o, T const& v) {
     ^
/home/dchappelle/environment/staging/include/msgpack/v1/adaptor/adaptor_base.hpp:65:6: note: candidate function [with T = std::basic_string<char>]
void operator<< (msgpack::object& o, T const& v) {
     ^
/home/dchappelle/environment/staging/include/msgpack/v2/adaptor/adaptor_base.hpp:60:6: note: candidate function [with T = std::basic_string<char>]
void operator<< (msgpack::object& o, T const& v) {
     ^
1 error generated.

Here is a simple program that reproduces the issue:

#include <iostream>
#include <string>
#include <msgpack.hpp>

class foo
{
public:
    foo() {}
    ~foo() {}

    const std::string& name() const { return m_name; }
    void set_name(const std::string& name) { m_name = name; }
private:
    std::string m_name;
};

namespace msgpack {
MSGPACK_API_VERSION_NAMESPACE(MSGPACK_DEFAULT_API_NS) {
namespace adaptor {

template<>
struct convert<foo>
{
    msgpack::object const& operator()(
            msgpack::object const& object,
            foo& f) const
    {
        f.set_name(object.as<std::string>());
        return object;
    }
};

template<>
struct pack<foo>
{
    template <typename Stream>
    msgpack::packer<Stream>& operator()(
            msgpack::packer<Stream>& packer,
            foo const& f) const
    {
        packer.pack(f.name());
        return packer;
    }
};

template <>
struct object_with_zone<foo>
{
    void operator()(
            msgpack::object::with_zone& object,
            const foo& f) const
    {
        object << f.name();
    }
};

} // namespace adaptor
} // MSGPACK_API_VERSION_NAMESPACE(MSGPACK_DEFAULT_API_NS)
} // namespace msgpack

int main(int argc, char** argv)
{
    foo f;
    msgpack::zone zone;
    msgpack::object object(f, zone);
    return 0;
}

If I use MSGPACK_API_VERSION_NAMESPACE(v1) then it compiles. So this is likely an issue with the inlining of the namespaces. I also went back through the commit history to find which commit introduced this problem since it was compiling before without issue until I updated msgpack. I found that the issue was introduced by this commit:

commit 54cb4350b3838e76b3fe117b0d1d5c75cdeb8da8
Author: Takatoshi Kondo <redboltz@gmail.com>
Date:   Fri Jan 22 21:44:58 2016 +0900

    Re-organized tree to prepare for version 2.0.0.

    See https://github.com/msgpack/msgpack-c/wiki/v1_1_cpp_versioning

[redboltz]

@davidchappelle , thank you for reporting the issue.

I sent #443 to fix the problem. Could you check it?

[davidchappelle]

Looks good. Thanks!

[redboltz]

Thank you for the checking. merged.