Line 137 in slot.h is part of sigc::internal::slot_call1::call_it().
Does the compiler tell you from where it is called, and with what parameters?
Does this happen in code that can be built with an earlier version of libsigc++?

A small test case would be appreciated.

(In reply to Kjell Ahlstedt from comment #1)
> Line 137 in slot.h is part of sigc::internal::slot_call1::call_it().
> Does the compiler tell you from where it is called, and with what parameters?
> Does this happen in code that can be built with an earlier version of
> libsigc++?

Hi Kjell.

The error happens when compiling "ardour" (https://ardour.org/). It used to work with previous version of libsigc++.
I will try to come up with a test case, but c++ is really _not_ my thing...
Thanks.

That's the entire error message:

g++ -o libs/gtkmm2ext/gtk_ui.os -c -O2 -pipe -std=c++11 -Woverloaded-virtual -DGTK_NEW_TOOLTIP_API -DPACKAGE=\"libgtkmm2ext\" -DLIBSIGC_DISABLE_DEPRECATED -O3 -fomit-frame-pointer -ffast-math -fstrength-reduce -pipe -DARCH_X86 -DBUILD_SSE_OPTIMIZATIONS -DUSE_X86_64_ASM -Wall -DHAVE_LIBLO -DPROGRAM_NAME=\"Ardour\" -D_REENTRANT -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D__STDC_FORMAT_MACROS -Ilibs -DENABLE_NLS -pthread -pthread -pthread -D_REENTRANT -DLOCALEDIR=\"/usr/local/share/locale\" -fPIC -Ilibs/pbd -I/usr/local/include/sigc++-2.0 -I/usr/local/include/gio-unix-2.0 -I/usr/local/include/gtkmm-2.4 -I/usr/local/include/cairomm-1.0 -I/usr/local/lib/pangomm-1.4/include -I/usr/local/include/pangomm-1.4 -I/usr/local/lib/glibmm-2.4/include -Ilibs/gtkmm2ext -I/usr/local/include/harfbuzz -I/usr/local/lib/sigc++-2.0/include -I/usr/local/include/gdk-pixbuf-2.0 -I/usr/local/include/gtk-2.0 -I/usr/local/include/gdkmm-2.4 -I/usr/local/lib/gtk-2.0/include -I/usr/local/include -I/usr/local/include/giomm-2.4 -I/usr/local/include/atkmm-1.6 -I/usr/X11R6/include/freetype2 -I/usr/local/include/libpng16 -I/usr/X11R6/include/pixman-1 -I/usr/local/include/gtk-unix-print-2.0 -I/usr/local/include/cairo -I/usr/local/lib/cairomm-1.0/include -I/usr/local/include/glib-2.0 -I/usr/local/lib/giomm-2.4/include -I/usr/include -I/usr/X11R6/include -I/usr/local/include/glibmm-2.4 -I/usr/local/lib/gdkmm-2.4/include -I/usr/local/lib/glib-2.0/include -I/usr/local/include/pango-1.0 -I/usr/local/lib/gtkmm-2.4/include -I/usr/local/include/atk-1.0 libs/gtkmm2ext/gtk_ui.cc
In file included from /usr/local/include/sigc++-2.0/sigc++/signal_base.h:27:0,
                 from /usr/local/include/sigc++-2.0/sigc++/signal.h:8,
                 from /usr/local/include/sigc++-2.0/sigc++/sigc++.h:86,
                 from /usr/local/include/glibmm-2.4/glibmm/thread.h:51,
                 from /usr/local/include/glibmm-2.4/glibmm.h:87,
                 from /usr/local/include/gtkmm-2.4/gtkmm.h:87,
                 from libs/gtkmm2ext/gtk_ui.cc:29:
/usr/local/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 = std::_Bind<sigc::pointer_functor2<_GdkEventAny*, Gtk::Window*, bool>(Gtk::Window*)>; T_return = bool; T_arg1 = _GdkEventAny*; sigc::type_trait_take_t<T_arg3> = _GdkEventAny* const&]':
/usr/local/include/sigc++-2.0/sigc++/functors/slot.h:144:37:   required from 'static void* (* sigc::internal::slot_call1<T_functor, T_return, T_arg1>::address())(void*) [with T_functor = std::_Bind<sigc::pointer_functor2<_GdkEventAny*, Gtk::Window*, bool>(Gtk::Window*)>; T_return = bool; T_arg1 = _GdkEventAny*; sigc::internal::hook = void* (*)(void*)]'
/usr/local/include/sigc++-2.0/sigc++/functors/slot.h:543:91:   required from 'sigc::slot1<T_return, T_arg1>::slot1(const T_functor&) [with T_functor = std::_Bind<sigc::pointer_functor2<_GdkEventAny*, Gtk::Window*, bool>(Gtk::Window*)>; T_return = bool; T_arg1 = _GdkEventAny*]'
/usr/local/include/sigc++-2.0/sigc++/functors/slot.h:1273:26:   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 = std::_Bind<sigc::pointer_functor2<_GdkEventAny*, Gtk::Window*, bool>(Gtk::Window*)>; T_return = bool; T_arg1 = _GdkEventAny*]'
libs/gtkmm2ext/gtk_ui.cc:103:95:   required from here
/usr/local/include/sigc++-2.0/sigc++/functors/slot.h:137:20: error: void value not ignored as it ought to be
                (a_1);
                    ^

I'm confused by this part of the compiler output:

  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>)

T_arg3 is surprising. It should be T_arg1. Is it possible that your copy of
/usr/local/include/sigc++-2.0/sigc++/functors/slot.h is damaged? Probably at
line 132. I doubt it, but it's worth checking. When I replaced T_arg1 by T_arg3
in slot.h:132, I got the more reasonable error message
  'T_arg3' was not declared in this scope


An aside: One of your compiler options is -DLIBSIGC_DISABLE_DEPRECATED.
You should change it to -DSIGCXX_DISABLE_DEPRECATED.

Until the year 2007, deprecated code was marked with LIBSIGC_DISABLE_DEPRECATED.
Then all deprecated code was removed. When I deprecated some other code in
2013, I marked it with SIGCXX_DISABLE_DEPRECATED. That's unfortunate. When I
introduced SIGCXX_DISABLE_DEPRECATED, I was not aware that
LIBSIGC_DISABLE_DEPRECATED had been used 6 years before.

(In reply to Kjell Ahlstedt from comment #4)
> I'm confused by this part of the compiler output:
> 
>   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>)
> 
> T_arg3 is surprising. It should be T_arg1. Is it possible that your copy of
> /usr/local/include/sigc++-2.0/sigc++/functors/slot.h is damaged? Probably at
> line 132. I doubt it, but it's worth checking. When I replaced T_arg1 by
> T_arg3
> in slot.h:132, I got the more reasonable error message
>   'T_arg3' was not declared in this scope

I think the file is fine (line 132 and above):

  static T_return call_it(slot_rep* rep, type_trait_take_t<T_arg1> a_1)
    {
      typedef typed_slot_rep<T_functor> typed_slot;
      typed_slot *typed_rep = static_cast<typed_slot*>(rep);
      return (typed_rep->functor_).SIGC_WORKAROUND_OPERATOR_PARENTHESES<type_trait_take_t<T_arg1>>
               (a_1);
    }


> An aside: One of your compiler options is -DLIBSIGC_DISABLE_DEPRECATED.
> You should change it to -DSIGCXX_DISABLE_DEPRECATED.
> 
> Until the year 2007, deprecated code was marked with
> LIBSIGC_DISABLE_DEPRECATED.
> Then all deprecated code was removed. When I deprecated some other code in
> 2013, I marked it with SIGCXX_DISABLE_DEPRECATED. That's unfortunate. When I
> introduced SIGCXX_DISABLE_DEPRECATED, I was not aware that
> LIBSIGC_DISABLE_DEPRECATED had been used 6 years before.

Ah, that's good to know, thanks for the hint :-)

It will be difficult to fix this bug without a test case. All test cases in the
libsigc++2/tests directory compile just fine. Perhaps ardour uses libsigc++ in
an unusual way. If you have problems making a small test case, perhaps I can
get some idea, if I see the code in libs/gtkmm2ext/gtk_ui.cc:137 where
sigc::slot is called. Is the code available on the web? Can you show a link?

Another aside: You should update ardour to use gtkmm 3 instead of gtkmm 2.

(In reply to Kjell Ahlstedt from comment #6)
> It will be difficult to fix this bug without a test case. All test cases in
> the
> libsigc++2/tests directory compile just fine. Perhaps ardour uses libsigc++
> in
> an unusual way. If you have problems making a small test case, perhaps I can
> get some idea, if I see the code in libs/gtkmm2ext/gtk_ui.cc:137 where
> sigc::slot is called. Is the code available on the web? Can you show a link?

Hi Kjell.

Yeah, I'll do my homework and try to provide better info.

> Another aside: You should update ardour to use gtkmm 3 instead of gtkmm 2.

That's actually the first thing I'm going to try. Maybe it'll help.
I'll get back to you as soon as I have something :-)

Thanks again.

Ok, it looks like adding:
namespace sigc { SIGC_FUNCTORS_HAVE_RESULT_TYPE }
fixes that particular issue.
I think ardour code is a bit old, I'll have a look with upstream.

Thanks again for all the pointers Kjell.