json::value_t can't be a map's key type in VC++ 2015

[Getfree]

The following code does not compile in Visual Studio 2015, but compiles fine in GCC.

#include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;
	
	map<JSON::value_t,string> jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

Wandbox: http://melpon.org/wandbox/permlink/Izdt8DW4UGpVQdUd

[nlohmann]

Can you please provide the error messages from MSVC?

[Getfree]

No problem. Here:

Z:\>cl /EHsc test3.cpp
test3.cpp
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(239): error C2593: 'operator <' is ambiguous
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(436): note: could be 'bool std::operator <(const std::error_condition &,const std::error_condition &) noexcept'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(427): note: or       'bool std::operator <(const std::error_code &,const std::error_code &) noexcept'
z:\json.hpp(171): note: or       'bool nlohmann::detail::operator <(const nlohmann::detail::value_t,const nlohmann::detail::value_t) noexcept' [found using argument-dependent lookup]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(239): note: while trying to match the argument list '(const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t, const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t)'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(238): note: while compiling class template member function 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const'
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\map(210): note: see reference to function template instantiation 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\type_traits(421): note: see reference to class template instantiation 'std::less<_Kty>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xutility(265): note: see reference to class template instantiation 'std::is_empty<_Ty1>' being compiled
        with
        [
            _Ty1=std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xtree(957): note: see reference to class template instantiation 'std::_Tree_comp_alloc<_Traits>' being compiled
        with
        [
            _Traits=std::_Tmap_traits<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string,std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>,std::allocator<std::pair<const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string>>,false>
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\map(73): note: see reference to class template instantiation 'std::_Tree<std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,false>>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=std::string,
            _Pr=std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>,
            _Alloc=std::allocator<std::pair<const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string>>
        ]
test3.cpp(11): note: see reference to class template instantiation 'std::map<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=std::string
        ]

[nlohmann]

Could you try explicitly to pass nlohmann::detail::operator< as Compare template argument to the map?

[Getfree]

I don't know how to pass the operator itself, but I tried passing a functor, like this:

#include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;

	struct value_tCompare {
		bool operator()(const JSON::value_t& a, const JSON::value_t& b) const{
			return a < b ;
		}
	} ;
	
	map<JSON::value_t, string, value_tCompare > jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

And the result was this:

Z:\>cl /EHsc test3.cpp
test3.cpp
test3.cpp(13): error C2593: 'operator <' is ambiguous
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(436): note: could be 'bool std::operator <(const std::error_condition &,const std::error_condition &) noexcept'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(427): note: or       'bool std::operator <(const std::error_code &,const std::error_code &) noexcept'
z:\json.hpp(171): note: or       'bool nlohmann::detail::operator <(const nlohmann::detail::value_t,const nlohmann::detail::value_t) noexcept' [found using argument-dependent lookup]
test3.cpp(13): note: while trying to match the argument list '(const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t, const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t)'

Then I tried using nlohmann::detail::operator< explicitly inside the functor, and it compiled successfully.

#include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;

	struct value_tCompare {
		bool operator()(const JSON::value_t& a, const JSON::value_t& b) const{
			return nlohmann::detail::operator<(a,b) ;
		}
	} ;
	
	map<JSON::value_t, string, value_tCompare > jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

This seems to be an argument-dependent lookup issue. For some reason the compiler is not prioritizing nlohmann::detail::operator< over the others.

Hopefully there is a way to solve it, so we don't need this workaround.

[nlohmann]

This seems to be an issue where @theodelrieu may know more.

[theodelrieu]

I will take a look tomorrow, this looks like a MSVC-specific weirdness at first sight

[theodelrieu]

There is a workaround mentioned in this StackOverflow thread
Should we add this to the library?

namespace std {
template <>
struct less<::nlohmann::detail::value_t>
{
  bool operator()(::nlohmann::detail::value_t lhs, ::nlohmann::detail::value_t rhs) const noexcept
  {
    return ::nlohmann::detail::operator<(lhs, rhs);
  }
};
}

[nlohmann]

Is it safe to add this to the std namespace? @gregmarr ?

[theodelrieu]

That's a template specialization, it's allowed and safe.

Plus, this is what the default std::less<nlohmann::detail::value_t> looks like on the other platforms.

[nlohmann]

The following code now compiles with MSVC 2015 (see https://ci.appveyor.com/project/nlohmann/json/build/1765):

std::map<json::value_t, std::string> jsonTypes ;
jsonTypes[json::value_t::array] = "array";

@Getfree Can you try this version of the code please: f4126e4

[Getfree]

I added the fixed version to my project and it compiled successfully.

Many thanks.