e:\code\foo.cpp(19): error C2593: 'operator =' is ambiguous
    d:\visual studio 2013\vc\include\map(212): could be 'std::map<std::string,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>> &std::map<_Kty,_Ty,std::less<_Ty>,std::allocator<std::pair<const _Kty,_Ty>>>::operator =(std::initializer_list<std::pair<const _Kty,_Ty>>)'
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=std::string
1>          ]
1>          d:\visual studio 2013\vc\include\map(166): or       'std::map<std::string,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>> &std::map<_Kty,_Ty,std::less<_Ty>,std::allocator<std::pair<const _Kty,_Ty>>>::operator =(std::map<_Kty,_Ty,std::less<_Ty>,std::allocator<std::pair<const _Kty,_Ty>>> &&)'
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=std::string
1>          ]
1>          d:\visual studio 2013\vc\include\map(150): or       'std::map<std::string,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>> &std::map<_Kty,_Ty,std::less<_Ty>,std::allocator<std::pair<const _Kty,_Ty>>>::operator =(const std::map<_Kty,_Ty,std::less<_Ty>,std::allocator<std::pair<const _Kty,_Ty>>> &)'
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=std::string
1>          ]
1>          while trying to match the argument list '(std::map<std::string,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>, initializer-list)'
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=std::string
1>          ]