The Gtk.Clipboard.set_with_data method currently (Vala 0.38.4) does not accept user_data argument. The signarure of the C function is:

gboolean
gtk_clipboard_set_with_data (GtkClipboard *clipboard,
                             const GtkTargetEntry *targets,
                             guint n_targets,
                             GtkClipboardGetFunc get_func,
                             GtkClipboardClearFunc clear_func,
                             gpointer user_data);


Call generated by vala:

gtk_clipboard_set_with_data (clipboard, targets, targets_length1, ____lambda7__gtk_clipboard_get_func, ____lambda8__gtk_clipboard_clear_func);


Leads to error when compiling is "error: too few arguments to function 'gtk_clipboard_set_with_data'".

The quite similar Gtk.Clipboard.set_with_owner seems to be allright.