According to these declarations:

class Item_basic_constant :public Item

{

  table_map used_table_map;

  ...

};

...

class Item_cache: public Item_basic_constant

{

protected:

  Item *example;

  table_map used_table_map;

...

}

Item_cache has two used_tables_cache, one of which is declared in Item_cache itself, and another one is derived from Item_basic_constant.

It's very likely that the second declaration is not really necessary.