Argument dependent lookup with stream operator for std::vector - c++

Question

I have created a stream output operator for std::vector in my namespace, and I was able to use that in my previous debug code:
namespace my_company {
template <typename T>
std::ostream &operator<<(std::ostream &os, std::vector<T> const &vec) {
os << "{";
for (int i = 0; i < vec.size(); ++i) {
if (i != 0) {
os << ", ";
}
os << vec[i];
}
os << "}";
return os;
}
template <typename T>
void debug_impl(T const &var, char const *const expr, char const *const file, int const line) {
using std::vector;
std::lock_guard lg(debug_print_mutex);
std::cout << "DEBUG " << file << ":" << line << " T" << std::this_thread::get_id() << "\n " << expr << ": "
<< var << std::endl;}
#define DEBUG(var) ::my_company::debug_impl(var, #var, __FILE__, __LINE__)
}
But now I want to use my logger class:
namespace my_company {
class Logger {
public:
virtual ~Logger() = default;
virtual void log_impl(LogLevel log_level, std::string message) = 0;
template <typename... T>
void log_bits(LogLevel log_level, T const &...ts) {
if (log_level >= threshold)
log_impl(log_level, format_bits(ts...));
}
template <typename... T>
void trace(T const &...ts) {
log_bits(LogLevel::trace, ts...);
}
private:
LogLevel threshold = LogLevel::trace;
};
}
The format function is this one:
namespace my_company {
template <typename... T>
std::string format_bits(T const &...ts) {
std::ostringstream oss;
// The following is a C++17 [fold expression](https://en.cppreference.com/w/cpp/language/fold).
(oss << ... << ts);
return oss.str();
}
}
In the debug_impl function I just delegate to the logger now:
namespace my_company {
template <typename T>
void debug_impl(T const &var, char const *const expr, char const *const file, int const line) {
logger->template trace(file, ":", line, " T", std::this_thread::get_id(), "\n ", expr, ": ", var);
}
}
Clang doesn't like that:
logger.h:29:11: error: call to function 'operator<<' that is neither visible in the template definition nor found by argument-dependent lookup
(oss << ... << ts);
^
logger.h:44:27: note: in instantiation of function template specialization 'my_company::format_bits<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
log_impl(log_level, format_bits(ts...));
^
logger.h:69:5: note: in instantiation of function template specialization 'my_company::Logger::log_bits<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
log_bits(LogLevel::trace, ts...);
^
util.h:693:20: note: in instantiation of function template specialization 'my_company::Logger::trace<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
logger->template trace(file, ":", line, " T", std::this_thread::get_id(), "\n ", expr, ": ", var);
^
rotating_buffer.h:40:5: note: in instantiation of function template specialization 'my_company::debug_impl<std::vector<bool, std::allocator<bool>>>' requested here
DEBUG(used);
^
util.h:699:42: note: expanded from macro 'DEBUG'
#define DEBUG(var) ::my_company::debug_impl(var, #var, __FILE__, __LINE__)
^
logger.h:13:15: note: 'operator<<' should be declared prior to the call site
std::ostream &operator<<(std::ostream &os, std::vector<T> const &vec) {
^
I have already declared that prior to the call site, it is all declared in the logger.h file, which is included by the others. The operator<< is the first thing in that file, so it should be available. I presume that the additional layer of templates did not make it any easier.
I have read a few other questions, but I don't understand how I can solve this issue as putting the operator<< into std isn't allowed, but it doesn't work in my_company and putting it into the global namespace doesn't work either. What can I do?
We use Clang 11 and C++20 on Linux.
Somewhat related:
Argument Dependent Lookup and stream operators overloading
Output stream operator Argument Dependent Lookup (ADL) for fundamental/STL types/classes

Answer 1

It seems to be a clang 11 bug.
Clang 12 accept the code Demo.
As work around, you might use, instead of (oss << ... << ts);:
((oss << ts), ...);