Add check for void return in typed functions

phases/typecheck.ml

@@ -186,6 +186,10 @@ let rec typecheck node =
           "function \"%s\" has return type %s, got %s"
           name (type2str ret_type) (type2str (typeof value)))]
 
+      | Return (None, _) as ret when ret_type <> Void ->
+        ret, [NodeMsg (ret, sprintf "expected return value of type %s"
+          (type2str ret_type))]
+
       | node -> traverse_list check_types node
     in
     let err =