Eiffel: compilation error `Source of assignment is not compatible with target`

Question

With complete void check set in compiler I've got a Variable is not properly set compilation error on following case which for me is right (in my mind). It says that the source of assignment is not compatible with target. What am I missing here??? ( DB_SERVICE.load_from_primary_key... )

Class DB_SERVICE

deferred class
    DB_SERVICE [G -> DB_ENTITY create make_from_db_result end]

inherit
    ACTION
        redefine
            start,
            execute
        end

    LOGGABLE
        rename
            make as make_from_loggable
        end

feature -- Creation

    make (a_db_connection: attached DB_CONNECTION)
        require
            valid_db_connection: a_db_connection.is_connected
        do
            make_from_loggable
            db_connection := a_db_connection
            create last_items.make (100)
            create last_column_names.make_empty
        ensure
            db_connection_setted: a_db_connection = db_connection and db_connection.is_connected
        end

feature -- Access

    item: detachable G

    db_connection: DB_CONNECTION

    last_items: HASH_TABLE[like item, INTEGER] -- content of last resultset

    last_column_names: ARRAY[STRING] -- Column names of last resultset

feature -- status_report


    load_from_primary_key (primary_key: INTEGER)
            -- Loads given item into item otherwise item will be Void
        require
            attached db_connection.base_selection
        local
            l_db_result: DB_RESULT
        do
            if attached db_connection.base_selection as bs then
                bs.query ("SELECT * FROM " + ({attached like item}).out + " WHERE " + {attached like item}.Primary_key_db_column_name + "=" + primary_key.out)
                if bs.is_ok then
                    bs.load_result
                    create item.make_from_db_result(last_column_names, bs.cursor)
                else
                    item := Void --HERE is the compiler complaining!
                    logger.write_critical ("Error while retreiving " + ({like item}).out + " from DB")
                end
            else
                item := Void 
                logger.write_error ("base_selection is void")
            end
        end


end -- class

Class COMPANY_SERVICE

class
    COMPANY_SERVICE

inherit
    DB_SERVICE[COMPANY]
        redefine
            make
        end
...

Class COMPANY

class
    COMPANY

inherit
    DB_ENTITY
        rename
            primary_key as id,
            set_primary_key as set_id,
            Primary_key_db_column_name as Id_db_column_name
        redefine
            make,
            make_from_db_result,
            out
        end

create
    make,
    make_from_db_result

....

Answer 1

The type declaration detachable G indicates that if the corresponding actual generic is a reference type, the variable of that type could be detachable. In that case it would be OK to assign Void to such a variable. However, it's also possible that the actual generic is an expanded type. Prefixing an expanded type with detachable has no effect, the type remains the same and the variable cannot be assigned Void .

As an example, let's consider a simpler case, when there is no formal generic constraint for the parameter G . An actual generic could be STRING , and the variable item has a type detachable STRING . In this case, it's OK to assign Void to item .

Now, if the actual generic is INTEGER , the variable has a type detachable INTEGER that is equivalent to INTEGER . Assigning Void to a variable of this type makes no sense and is not permitted by the language rules.

The variable item still can be set to Void if the actual generic parameter is a reference type. For that, a local variable with the same type can be declared and its value can be assigned to item :

local
    default_item: like item
do
    item := default_item