All the operator<< for QDebug should be hidden friends ideally.

The result of not having them be hidden friends is that the following compiles:

#include <qdebug.h>
#include <qstring.h>
#include <vector>
void foo() {
    QString *ptr = nullptr;
    ptr << std::vector<int>{}
}

But it doesn't compile (which it shouldn't), if you comment out the include of qdebug.h.