Oracle 18c

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CREATE TYPE parent_type IS OBJECT (
 x NUMBER,
 y NUMBER
) NOT FINAL;
CREATE TYPE child_type UNDER parent_type (
  MEMBER FUNCTION get_x RETURN NUMBER
);
CREATE TYPE BODY child_type IS
  MEMBER FUNCTION get_x RETURN NUMBER
  IS
  BEGIN
    RETURN self.x;
  END;
END;
/
CREATE FUNCTION create_parent RETURN PARENT_TYPE
IS
BEGIN
  RETURN child_type(1, 2);
END;
/
SELECT create_parent().get_x() FROM DUAL;
ORA-00904: "FIDDLE_YWRJUALSLSGRNKFNQKMT"."PARENT_TYPE"."GET_X": invalid identifier

SELECT TREAT(create_parent() AS child_type).get_x() FROM DUAL;


TREAT(CREATE_PARENT()ASCHILD_TYPE).GET_X()


1