c++ - Problems with new QFlags helpers for qDebug() in QT5.5 -

i have following code

#include <qdebug>  //needed other stuff!! #include <qdatastream>  class testclass { public:     class subclass {     public:         qstring a;         int b;          /*         //needed qdatastream not relevant generated error         friend qdatastream& operator<<(qdatastream& os, subclass const& f)         {             os << f.a;             os << f.b;             return os;         }          friend qdatastream& operator>>(qdatastream& is, subclass& f)         {             >> f.a;             >> f.b;             return is;         }*/          friend qdatastream& operator<< <subclass>(qdatastream&, qlist<subclass> const&);         friend qdatastream& operator>> <subclass>(qdatastream&, qlist<subclass>&);      }; }; 

compiling qt5.5 gives me following errors:

/usr/include/qt/qtcore/qflags.h:88: error: invalid application of 'sizeof' incomplete type 'testclass::subclass'     q_static_assert_x((sizeof(enum) <= sizeof(int)),                    ^~~~~~~~~~~~ /usr/include/qt/qtcore/qtypetraits.h:478: error: 'testclass::subclass' incomplete type     : integral_constant<bool, (t(0) < t(-1))> {};                                ^ /usr/include/qt/qtcore/qdebug.h:279: in instantiation of template class 'qtprivate::isqenumhelper<qflags<testclass::subclass> >' requested here     qtprivate::isqenumhelper<t>::value || qtprivate::isqenumhelper<qflags<t> >::value,                                                      ^ main.cpp:25: while substituting explicitly-specified template arguments function template 'operator<<'         friend qdatastream& operator<< <subclass>(qdatastream&, qlist<subclass> const&);                             ^ 

the reason seems lie in following code of qdebug header file (qdebug.h:277):

template <class t> inline typename qtprivate::qenableif<     qtprivate::isqenumhelper<t>::value || qtprivate::isqenumhelper<qflags<t> >::value,     qdebug>::type operator<<(qdebug debug, const qflags<t> &flags) {   //... } 

compiling qt5.4 works fine. right template function taken (qdatastream.h:241)

template <typename t> qdatastream& operator<<(qdatastream& s, const qlist<t>& l) {    //... } 

questions:

1. i know why template qdebug.h taken consideration. have ideas, answer appropriate links c++ standard , proper explanation nice.
2. is there workaround or should submit bugreport qt?

the following (simplified code) generates same error.

#include <qdebug>  //needed cause error  class testclassa {}; class testclassb {};  template <typename t> testclassa& operator<<(testclassa& s, t& l) {     q_unused(l)     return s; }  template testclassa& operator<< <testclassb>(testclassa& s, testclassb& l); 

bugreport opened at: https://bugreports.qt.io/browse/qtbug-47375

update: fixed it. fix included in qt 5.5.1