With the following UI definition:

----------UI definition----
  <template class="BoxesUnattendedSetupBox" parent="GtkBox">
...
            <child>
              <object class="GtkEntry" id="username_entry">
                <signal name="notify::text" handler="on_mandatory_input_changed"/>
                <signal name="activate" handler="on_username_entry_activated"/>
              </object>

---------------------------

and this code:

----------CODE-------------
[GtkTemplate (ui = "/org/gnome/Boxes/ui/unattended-setup-box.ui")]
private class Boxes.UnattendedSetupBox : Gtk.Box {
..
    [GtkCallback]
    private void on_mandatory_input_changed () {
        notify_property ("ready-to-create");
    }

    [GtkCallback]
    private void on_username_entry_activated () {
        if (ready_for_express)
            user_wants_to_create ();
        else if (username != "" && product_key_format != null)
            product_key_entry.grab_focus (); // If username is provided, must be product key thats still not provided
    }
-------------------------

I get an error from valac:

unattended-setup-box.vala:109.5-109.43: error: could not find signal for handler `on_mandatory_input_changed'
    private void on_mandatory_input_changed () {
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

However there is no warning about handlers of signals that do not have details, e.g 'on_username_entry_activated' above.