[SOLVED] QT5 Radio button signal not compatible with function, even though it should be

According to the QT5 docs, the QRadioButton inherits the 4 signals from QAbstractButton, which includes the clicked signal. The template for it is clicked(bool checked = false) so I'm confused why this code returns a long error message, part of which is error: static assertion failed: Signal and slot arguments are not compatible.

Code (Simplified, may contain mistakes that are not part of the original):

#include <QApplication>
#include <QMainWindow>
#include <QRadioButton>

#include <iostream>

class front_end : public QWidget
{
  Q_OBJECT

public:
  void load()
  {
    auto *rbtn_0 = new QRadioButton(tr("Reference"), this);
    rbtn_0->setChecked(true);

    QObject::connect(rbtn_0, &QRadioButton::clicked, this, SLOT(rbtn_toggle(bool)));
  }

public slots:
  void rbtn_toggle(bool arg_0)
  {
    std::cout << "Toggle\n";
  }
};

int main(int argc, char *argv[]) {
  QApplication app(argc, argv);

  front_end FrontEnd;
  FrontEnd.load();

  return app.exec();
}

Compiled with:

qmake -project -norecursive -Wall -Wparser -o main.pro;
echo "QT += widgets" >> main.pro;
echo "CONFIG += c++11" >> main.pro;

qmake -norecursive main.pro;
make;

Error Message:

g++ -c -m64 -pipe -O2 -std=c++0x -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 -I. -I. -I/usr/include/qt5 -I/usr/include/qt5/QtWidgets -I/usr/include/qt5/QtGui -I/usr/include/qt5/QtCore -I. -o main.o main.cpp
main.cpp:21:8: warning: unused parameter ‘arg_0’ [-Wunused-parameter]
   void rbtn_toggle(bool arg_0)
        ^
In file included from /usr/include/qt5/QtCore/qcoreapplication.h:48:0,
                 from /usr/include/qt5/QtWidgets/qapplication.h:45,
                 from /usr/include/qt5/QtWidgets/QApplication:1,
                 from main.cpp:1:
/usr/include/qt5/QtCore/qobject.h: In instantiation of ‘static typename QtPrivate::QEnableIf<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == (-1)), QMetaObject::Connection>::Type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const QObject*, Func2, Qt::ConnectionType) [with Func1 = void (QAbstractButton::*)(bool); Func2 = const char*; typename QtPrivate::QEnableIf<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == (-1)), QMetaObject::Connection>::Type = QMetaObject::Connection; typename QtPrivate::FunctionPointer<Func>::Object = QAbstractButton]’:
main.cpp:17:83:   required from here
/usr/include/qt5/QtCore/qobject.h:300:122: warning: overflow in implicit constant conversion [-Woverflow]
         const int FunctorArgumentCount = QtPrivate::ComputeFunctorArgumentCount<Func2 , typename SignalType::Arguments>::Value;
                                                                                                                          ^
In file included from /usr/include/qt5/QtCore/qcoreapplication.h:45:0,
                 from /usr/include/qt5/QtWidgets/qapplication.h:45,
                 from /usr/include/qt5/QtWidgets/QApplication:1,
                 from main.cpp:1:
/usr/include/qt5/QtCore/qglobal.h:669:47: error: static assertion failed: Signal and slot arguments are not compatible.
 #define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)
                                               ^
/usr/include/qt5/QtCore/qobject.h:302:9: note: in expansion of macro ‘Q_STATIC_ASSERT_X’
         Q_STATIC_ASSERT_X((FunctorArgumentCount >= 0),
         ^
In file included from /usr/include/qt5/QtCore/qobjectdefs.h:47:0,
                 from /usr/include/qt5/QtCore/qobject.h:48,
                 from /usr/include/qt5/QtCore/qcoreapplication.h:48,
                 from /usr/include/qt5/QtWidgets/qapplication.h:45,
                 from /usr/include/qt5/QtWidgets/QApplication:1,
                 from main.cpp:1:
/usr/include/qt5/QtCore/qobjectdefs_impl.h: In instantiation of ‘struct QtPrivate::FunctorReturnType<const char*, QtPrivate::List<> >’:
/usr/include/qt5/QtCore/qobject.h:305:158:   required from ‘static typename QtPrivate::QEnableIf<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == (-1)), QMetaObject::Connection>::Type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const QObject*, Func2, Qt::ConnectionType) [with Func1 = void (QAbstractButton::*)(bool); Func2 = const char*; typename QtPrivate::QEnableIf<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == (-1)), QMetaObject::Connection>::Type = QMetaObject::Connection; typename QtPrivate::FunctionPointer<Func>::Object = QAbstractButton]’
main.cpp:17:83:   required from here
/usr/include/qt5/QtCore/qobjectdefs_impl.h:633:78: error: request for member ‘operator()’ in ‘QtPrivate::FunctorReturnType<Functor, QtPrivate::List<Tail ...> >::dummy<const char*>()’, which is of non-class type ‘const char*’
         typedef decltype(dummy<Functor>().operator()((dummy<ArgList>())...)) Value;

Edit:

Yes, I know changing the signal syntax to the QT4 standard "SIGNAL(clicked(bool))" would fix it, but I'm trying to get away from that syntax, and more importantly, understand the problem.