When can symbol accessed from template be declared later [duplicate] - c++

Question

I am writing a simple logger class similar to QDebug, which has a template method that save data into QStringList. The code is here:
#include <QtCore/QString>
#include <QtCore/QStringList>
#include <QtCore/QTextStream>
class Logger
{
public:
Logger();
~Logger();
template <typename V>
Logger &operator<<(V const &value);
private:
QStringList msg;
};
inline Logger::Logger():
msg(QString("INFO:"))
{}
inline Logger::~Logger()
{
QTextStream out(stderr);
out << msg.join("");
}
template <typename V>
inline Logger &Logger::operator<<(V const &value)
{
msg << log(value);
return *this;
}
inline QString log(QString const &value)
{
return value;
}
inline QString log(int const (&value)[20])
{
return QString("Array");
}
int main(int argc, char *argv[])
{
Logger c;
int a[20] = {};
c << QString("test") << a;
return 0;
}
However, this doesn't compile with GCC 4.8.3.
$ g++ -I/usr/include/qt4 -L/usr/lib64/qt4 -lQtCore -o test2 test2.cpp
test2.cpp: In instantiation of ‘Logger& Logger::operator<<(const V&) [with V = int [20]]’:
test2.cpp:50:29: required from here
test2.cpp:32:21: error: ‘log’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
msg << log(value);
^
test2.cpp:41:16: note: ‘QString log(const int (&)[20])’ declared here, later in the translation unit
inline QString log(int const (&value)[20])
Indeed, if I move inline QString log(int const (&value)[20]) to the beginning or put a forward declaration, it compiles and works. But what confuses me is that inline QString log(QString const &value) works without any problem:
$ ./test2
INFO:testArray
I notice that QHash relies on qHash function, which is similar in this case. QHash works pretty fine with user-defined key classes (except arrays, which can't be a function return type).
Why do they behave differently? What did I miss here?
Thank you for your help.
BTW: Would you please tell me what would be good keywords for this question. I've tried combinations of "specialiation" "template" "forward declaration" "QHash" and "user-defined types", but they don't work.

Answer 1

The name log is looked up twice. At the point of template definition, ordinary lookup is performed. It doesn't find anything, since log is not declared at this point.
Then, at the point of instantiation, only argument-dependent lookup is performed. When the parameter is of type QString, the global namespace is searched since QString is declared there, and so log(QString) is found. But the type int[] doesn't have any associated namespaces, so argument-dependent lookup has nothing to search and finds nothing. Hence the error.