check_widget(): Also check .is_widget entry

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/base.lua

@@ -559,7 +559,8 @@ end
 --- Do some sanity checking on widget. This function raises a lua error if
 -- widget is not a valid widget.
 function base.check_widget(widget)
-    assert(type(widget) == "table")
+    assert(type(widget) == "table", "Type should be table, but is " .. tostring(type(widget)))
+    assert(widget.is_widget, "Argument is not a widget!")
     for _, func in pairs({ "add_signal", "connect_signal", "disconnect_signal" }) do
         assert(type(widget[func]) == "function", func .. " is not a function")
     end

spec/wibox/test_utils.lua

@@ -84,6 +84,7 @@ return {
         local w = object()
         w:add_signal("widget::redraw_needed")
         w:add_signal("widget::layout_changed")
+        w.is_widget = true
         w.visible = true
         w.opacity = 1
         if width or height then