glibmm 2.64.4 breaks gtkmm
I'm building the GNOME 3.38.2 release and I notice that glibmm 2.64.4 seems to be broken, or at least it's incompatible with the latesat gtkmm 3.24.2. I also notice that glibmm 2.64.3 does not exist, which is weird. Reverting to glibmm 2.64.2 fixes the gtkmm build, so I'm going to hold glibmm to 2.64.2. It would be great if this could be fixed by releasing either a corrected glibmm or gtkmm tarball sometime before the next GNOME tarball deadline (January 9).
../gtkmm/button.h: In constructor ‘Gtk::Button::Button(const Glib::ustring&, bool)’:
../gtkmm/button.h:85:23: error: ‘Gtk::Button::button_class_’ has incomplete type
85 | static CppClassType button_class_;
| ^~~~~~~~~~~~~
../gtkmm/button.h:39:9: note: forward declaration of ‘Gtk::Button::CppClassType’ {aka ‘class Gtk::Button_Class’}
39 | { class Button_Class; } // namespace Gtk
| ^~~~~~~~~~~~
(I haven't investigated to see what change caused that.)