Visual Studio 15：重载"<<" C++中的插入运算符

下面的代码在GNU GCC v7.1.1上工作正常，但在VS15
上给出一个错误

#include <iostream>
class Logger {
public:
//var should be initialized in  init loge level function
bool debug_log_level;
friend Logger& operator << (Logger& p, const char* str);
Logger& logs(std::string severity, bool level = 0);
Logger& getInstance();
};
Logger logger;
Logger& instance= logger;
Logger& Logger::getInstance() {
static Logger theInstance;
return theInstance;
}
Logger& Logger::logs(std::string severity, bool level)
{
//Unused(level) as init is one time activity and will use it's own variable.
std::cout<<severity << ": ";
return instance;
}
//will be invoked when RHS is class reference and LHS is "str"
Logger& operator<<(Logger& p, const char* str)
{
if(p.debug_log_level)
std::cout << str << std::endl;
return instance;
}
int main()
{
logger.debug_log_level=true;
logger.logs("Info",0)<<"Ravi..";
return 0;
}

VS15 给出错误：在std::cout<<severity << ": ";行的函数Logger::logs(std::string severity, bool level)中，没有运算符 "<<" 匹配这些操作数。
如果我缺少VS15特定的内容，请告诉我。