GtkApplication's session save/restore API (gtk_application_save(),
gtk_application_forget(), GtkRestoreReason, the "support-save" and
"autosave-interval" properties, and the "restore-window"/"save-state"/
"restore-state" signals) has been repeatedly added, removed, and
reverted upstream in GTK4 over the past year, and was removed again
on 2026-08-01 ("application: Drop public save/restore API"). gtkmm
4.23.0 still wraps it, which fails to build against a gtk without it:

  application.cc:279:86: error: unknown type name 'GtkRestoreReason';
  did you mean 'Gtk::RestoreReason'?
  application.cc:724:3: error: use of undeclared identifier
  'gtk_application_save'
  application.cc:729:3: error: use of undeclared identifier
  'gtk_application_forget'

Drop the wrapping until gtk settles on whether this API exists. As of
this patch, upstream gtkmm's git main hasn't reacted to gtk's latest
removal yet (checked 2026-08-03), so this isn't a backport of an
existing commit -- if upstream lands their own fix, prefer that
instead.

--- a/gtk/src/application.hg
+++ b/gtk/src/application.hg
@@ -345,28 +345,16 @@

   _WRAP_METHOD(Glib::RefPtr<Gio::Menu> get_menu_by_id(const Glib::ustring& id), gtk_application_get_menu_by_id, refreturn)
   _WRAP_METHOD(Glib::RefPtr<const Gio::Menu> get_menu_by_id(const Glib::ustring& id) const, gtk_application_get_menu_by_id, refreturn, constversion)
-  _WRAP_METHOD(void save(), gtk_application_save)
-  _WRAP_METHOD(void forget(), gtk_application_forget)
-
   _WRAP_PROPERTY("menubar", Glib::RefPtr<Gio::MenuModel>)
   _WRAP_PROPERTY("register-session", bool,
     deprecated "4.20: This property is ignored. GTK always registers with the session manager.")
   _WRAP_PROPERTY("screensaver-active", bool, newin "3,94")
   _WRAP_PROPERTY("active-window", Window*)
-  _WRAP_PROPERTY("support-save", bool)
-  _WRAP_PROPERTY("autosave-interval", unsigned int)
-
   _WRAP_SIGNAL(void window_added(Window* window), "window-added")
   _WRAP_SIGNAL(void window_removed(Window* window), "window-removed")
   // no_default_handler because the wrapped C signal has no default handler.
   _WRAP_SIGNAL(void query_end(), "query-end", no_default_handler)

-#m4 _CONVERSION(`GVariant*',`const Glib::VariantBase&',`Glib::wrap($3, true)')
-#m4 _CONVERSION(`GVariantDict*',`const Glib::RefPtr<Glib::VariantDict>&',`Glib::wrap($3, true)')
-  // TODO: Remove the following 3 no_default_handler when we can break ABI.
-  _WRAP_SIGNAL(void restore_window(RestoreReason reason, const Glib::VariantBase& state), "restore-window", no_default_handler)
-  _WRAP_SIGNAL(bool save_state(const Glib::RefPtr<Glib::VariantDict>& dict), "save-state", no_default_handler)
-  _WRAP_SIGNAL(bool restore_state(RestoreReason reason, const Glib::VariantBase& state), "restore-state", no_default_handler)

 private:
   /** This is just a way to call Glib::init() before calling a Glib::Object ctor,
--- a/gtk/src/enums.hg
+++ b/gtk/src/enums.hg
@@ -66,7 +66,6 @@
 _WRAP_ENUM(InterfaceColorScheme, GtkInterfaceColorScheme, decl_prefix GTKMM_API)
 _WRAP_ENUM(InterfaceContrast, GtkInterfaceContrast, decl_prefix GTKMM_API)
 _WRAP_ENUM(ReducedMotion, GtkReducedMotion, decl_prefix GTKMM_API)
-_WRAP_ENUM(RestoreReason, GtkRestoreReason, decl_prefix GTKMM_API)

 #ifndef DOXYGEN_SHOULD_SKIP_THIS
 //We need this because we can't just use floats for enum value.