The error is comming from this function.
inline T_return operator()(arg1_type_ _A_a1) const { if (!empty() && !blocked()) return (reinterpret_cast<call_type>(slot_base::rep_->call_))(slot_base::rep_, _A_a1); return T_return(); } /bin/sh ../../libtool --tag=CXX --mode=compile g++ -DHAVE_CONFIG_H -I. -I/home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm -I../.. -I../../webkit -I.. -I/home/dannick/Documents/programming/webkitmm/mainline/webkit -I../.. -DG_LOG_DOMAIN=\"webkitmm\" -pthread -I/usr/include/gtkmm-2.4 -I/usr/lib/gtkmm-2.4/include -I/usr/include/atkmm-1.6 -I/usr/include/gtk-unix-print-2.0 -I/usr/include/gtk-2.0 -I/usr/include/gdkmm-2.4 -I/usr/lib/gdkmm-2.4/include -I/usr/include/giomm-2.4 -I/usr/lib/giomm-2.4/include -I/usr/include/pangomm-1.4 -I/usr/lib/pangomm-1.4/include -I/usr/include/glibmm-2.4 -I/usr/lib/glibmm-2.4/include -I/usr/include/cairomm-1.0 -I/usr/lib/cairomm-1.0/include -I/usr/include/sigc++-2.0 -I/usr/lib/sigc++-2.0/include -I/usr/include/webkitgtk-1.0 -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/libdrm -I/usr/include/libpng15 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng15 -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/freetype2 -I/usr/include/libsoup-2.4 -I/usr/include/libxml2 -I/usr/include/webkitgtk-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -Wall -g -O2 -MT webinspector.lo -MD -MP -MF .deps/webinspector.Tpo -c -o webinspector.lo /home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm/webinspector.cc libtool: compile: g++ -DHAVE_CONFIG_H -I. -I/home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm -I../.. -I../../webkit -I.. -I/home/dannick/Documents/programming/webkitmm/mainline/webkit -I../.. -DG_LOG_DOMAIN=\"webkitmm\" -pthread -I/usr/include/gtkmm-2.4 -I/usr/lib/gtkmm-2.4/include -I/usr/include/atkmm-1.6 -I/usr/include/gtk-unix-print-2.0 -I/usr/include/gtk-2.0 -I/usr/include/gdkmm-2.4 -I/usr/lib/gdkmm-2.4/include -I/usr/include/giomm-2.4 -I/usr/lib/giomm-2.4/include -I/usr/include/pangomm-1.4 -I/usr/lib/pangomm-1.4/include -I/usr/include/glibmm-2.4 -I/usr/lib/glibmm-2.4/include -I/usr/include/cairomm-1.0 -I/usr/lib/cairomm-1.0/include -I/usr/include/sigc++-2.0 -I/usr/lib/sigc++-2.0/include -I/usr/include/webkitgtk-1.0 -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/libdrm -I/usr/include/libpng15 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng15 -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/freetype2 -I/usr/include/libsoup-2.4 -I/usr/include/libxml2 -I/usr/include/webkitgtk-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -Wall -g -O2 -MT webinspector.lo -MD -MP -MF .deps/webinspector.Tpo -c /home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm/webinspector.cc -fPIC -DPIC -o .libs/webinspector.o In file included from /usr/include/sigc++-2.0/sigc++/signal_base.h:29:0, from /usr/include/sigc++-2.0/sigc++/signal.h:8, from /usr/include/sigc++-2.0/sigc++/sigc++.h:80, from /usr/include/glibmm-2.4/glibmm/thread.h:58, from /usr/include/glibmm-2.4/glibmm.h:87, from /home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm/webinspector.cc:4: /usr/include/sigc++-2.0/sigc++/functors/slot.h: In instantiation of 'T_return sigc::slot1<T_return, T_arg1>::operator()(sigc::slot1<T_return, T_arg1>::arg1_type_) const [with T_return = WebKit::WebView&; T_arg1 = WebKit::WebView&; sigc::slot1<T_return, T_arg1>::arg1_type_ = WebKit::WebView&]': /home/dannick/Documents/programming/webkitmm/mainline/webkit/webkitmm/webinspector.cc:249:1: required from here /usr/include/sigc++-2.0/sigc++/functors/slot.h:516:23: error: value-initialization of reference type 'WebKit::WebView&' /usr/include/sigc++-2.0/sigc++/functors/slot.h: In member function 'T_return sigc::slot1<T_return, T_arg1>::operator()(sigc::slot1<T_return, T_arg1>::arg1_type_) const [with T_return = WebKit::WebView&; T_arg1 = WebKit::WebView&; sigc::slot1<T_return, T_arg1>::arg1_type_ = WebKit::WebView&]': /usr/include/sigc++-2.0/sigc++/functors/slot.h:517:5: warning: control reaches end of non-void function [-Wreturn-type] make[2]: *** [webinspector.lo] Error 1 |