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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, unlinks it from thd->free_list again (the copy shares pointers with the original and must not be destroyed) and asserts that the copy has the same typeid as the original. The copy is never destroyed: its destructor is not run, and its memory is released when the MEM_ROOT it was allocated on, the current thd->mem_root, is freed. A comment in the check says so. 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). Item_field_packable_row::shallow_copy() asserted to mark the class as never copied, which the check trips over since it copies every type it meets. It now returns nullptr, the existing way for an item to refuse a copy. No test is added: the check runs under every existing test in a debug build. main and the SQL-layer suites (compat/*, json, funcs_*, vcol, gcol, versioning, period, sys_vars, type_*) pass with it in both normal and --ps-protocol mode. This commit was prepared with Claude Code (Opus 5), which wrote the runtime type check, ran the main and SQL-layer suites in normal and --ps-protocol mode to find the classes that inherited a parent's copy method, added the ITEM_SHALLOW_COPY overrides for them, changed Item_field_packable_row to return nullptr, and documented where the copy made by the check is released.