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c894d97781
Item::shallow_copy() was meant to create a copy of the same class as the item it is called on, but nothing enforced that. A class that does not declare its own override inherits the one of its nearest concrete parent, and get_item_copy<Parent>() then silently creates an object of the parent class, losing the child's virtual behaviour (see MDEV-38801 for an earlier instance with Item_cache_year). Add the ITEM_SHALLOW_COPY(CLASS) macro, which declares the override, and Item::dbug_check_shallow_copy_type(), a debug-only check called from Query_arena::free_items() for every item freed. Once per dynamic type it creates a shallow copy and asserts that it has the same typeid as the original. Then, the copy is destroyed, which provides additional coverage. The check found these classes copied as their parent class, now fixed with ITEM_SHALLOW_COPY(): Item_join_operator_plus, Item_null_result, Item_old_field, Item_datetime, Item_empty_string, Item_return_int, Item_trigger_type_of_statement, in_string::Item_string_for_in_vector, Item_is_not_null_test, Item_interval_DDhhmmssff_typecast, Item_string_xml_non_const, Item_func_trt_trx_sees_eq, Item_field_refcursor (type_cursor plugin), Item_field_packable_scalar (type_assoc_array plugin). These two classes do not support copying so the patch makes their shallow_copy() to return NULL: Item_cache_wrapper, Item_sum_sp Item_field_packable_row::shallow_copy() has assert(0). The patch makes it return NULL. This commit was prepared with help of Claude Code (Opus 5). Sergei Petrunia added "delete item_copy" and did other minor edits.