g++ -O3 --std=c++17 `pkg-config --cflags gtkmm-3.0`   -c -o src/window_main.o src/window_main.cpp
In file included from /usr/include/sigc++-2.0/sigc++/signal_base.h:27,
                 from /usr/include/sigc++-2.0/sigc++/signal.h:8,
                 from /usr/include/sigc++-2.0/sigc++/sigc++.h:104,
                 from /usr/include/glibmm-2.4/glibmm/thread.h:49,
                 from /usr/include/glibmm-2.4/glibmm.h:88,
                 from /usr/include/gtkmm-3.0/gtkmm.h:87,
                 from src/window_main.h:21,
                 from src/window_main.cpp:1:
/usr/include/sigc++-2.0/sigc++/functors/slot.h: In instantiation of ‘static T_return sigc::internal::slot_call1<T_functor, T_return, T_arg1>::call_it(sigc::internal::slot_rep*, sigc::type_trait_take_t<T_arg3>) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*; sigc::type_trait_take_t<T_arg3> = _GdkEventKey* const&]’:
/usr/include/sigc++-2.0/sigc++/functors/slot.h:155:37:   required from ‘static void* (* sigc::internal::slot_call1<T_functor, T_return, T_arg1>::address())(void*) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*; sigc::internal::hook = void* (*)(void*)]’
/usr/include/sigc++-2.0/sigc++/functors/slot.h:657:90:   required from ‘sigc::slot1<T_return, T_arg1>::slot1(const T_functor&) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*]’
/usr/include/sigc++-2.0/sigc++/functors/slot.h:1821:26:   required from ‘sigc::slot<T_return, T_arg1, sigc::nil, sigc::nil, sigc::nil, sigc::nil, sigc::nil, sigc::nil>::slot(const T_functor&) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*]’
src/window_main.cpp:55:113:   required from here
/usr/include/sigc++-2.0/sigc++/functors/slot.h:148:20: error: void value not ignored as it ought to be
                (a_1);
                    ^
In file included from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7,
                 from /usr/include/sigc++-2.0/sigc++/signal_base.h:27,
                 from /usr/include/sigc++-2.0/sigc++/signal.h:8,
                 from /usr/include/sigc++-2.0/sigc++/sigc++.h:104,
                 from /usr/include/glibmm-2.4/glibmm/thread.h:49,
                 from /usr/include/glibmm-2.4/glibmm.h:88,
                 from /usr/include/gtkmm-3.0/gtkmm.h:87,
                 from src/window_main.h:21,
                 from src/window_main.cpp:1:
/usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h: In instantiation of ‘typename sigc::adaptor_functor<T_functor>::deduce_result_type<T_arg1>::type sigc::adaptor_functor<T_functor>::operator()(T_arg1) const [with T_arg1 = _GdkEventKey* const&; T_functor = sigc::bound_mem_functor0<void, WindowMain>; typename sigc::adaptor_functor<T_functor>::deduce_result_type<T_arg1>::type = void]’:
/usr/include/sigc++-2.0/sigc++/functors/slot.h:148:20:   required from ‘static T_return sigc::internal::slot_call1<T_functor, T_return, T_arg1>::call_it(sigc::internal::slot_rep*, sigc::type_trait_take_t<T_arg3>) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*; sigc::type_trait_take_t<T_arg3> = _GdkEventKey* const&]’
/usr/include/sigc++-2.0/sigc++/functors/slot.h:155:37:   required from ‘static void* (* sigc::internal::slot_call1<T_functor, T_return, T_arg1>::address())(void*) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*; sigc::internal::hook = void* (*)(void*)]’
/usr/include/sigc++-2.0/sigc++/functors/slot.h:657:90:   required from ‘sigc::slot1<T_return, T_arg1>::slot1(const T_functor&) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*]’
/usr/include/sigc++-2.0/sigc++/functors/slot.h:1821:26:   required from ‘sigc::slot<T_return, T_arg1, sigc::nil, sigc::nil, sigc::nil, sigc::nil, sigc::nil, sigc::nil>::slot(const T_functor&) [with T_functor = sigc::bound_mem_functor0<void, WindowMain>; T_return = bool; T_arg1 = _GdkEventKey*]’
src/window_main.cpp:55:113:   required from here
/usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:89:30: error: no match for call to ‘(sigc::bound_mem_functor0<void, WindowMain>) (_GdkEventKey* const&)’
     { return functor_(_A_arg1); }
                              ^
In file included from /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:9,
                 from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7,
                 from /usr/include/sigc++-2.0/sigc++/signal_base.h:27,
                 from /usr/include/sigc++-2.0/sigc++/signal.h:8,
                 from /usr/include/sigc++-2.0/sigc++/sigc++.h:104,
                 from /usr/include/glibmm-2.4/glibmm/thread.h:49,
                 from /usr/include/glibmm-2.4/glibmm.h:88,
                 from /usr/include/gtkmm-3.0/gtkmm.h:87,
                 from src/window_main.h:21,
                 from src/window_main.cpp:1:
/usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1990:12: note: candidate: ‘T_return sigc::bound_mem_functor0<T_return, T_obj>::operator()() const [with T_return = void; T_obj = WindowMain]’
   T_return operator()() const
            ^~~~~~~~
/usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1990:12: note:   candidate expects 0 arguments, 1 provided
In file included from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7,
                 from /usr/include/sigc++-2.0/sigc++/signal_base.h:27,
                 from /usr/include/sigc++-2.0/sigc++/signal.h:8,
                 from /usr/include/sigc++-2.0/sigc++/sigc++.h:104,
                 from /usr/include/glibmm-2.4/glibmm/thread.h:49,
                 from /usr/include/glibmm-2.4/glibmm.h:88,
                 from /usr/include/gtkmm-3.0/gtkmm.h:87,
                 from src/window_main.h:21,
                 from src/window_main.cpp:1:
/usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:89:30: error: return-statement with a value, in function returning ‘sigc::adaptor_functor<sigc::bound_mem_functor0<void, WindowMain> >::deduce_result_type<_GdkEventKey* const&, void, void, void, void, void, void>::type’ {aka ‘void’} [-fpermissive]
     { return functor_(_A_arg1); }
                              ^
make: *** [<builtin>: src/window_main.o] Error 1