--Here are my declarations. Note that there is more than one column in the record.
--The examples I could find online only use one column:
type mytable_rec is record
(
      mytable_col1 mytable.mytable_col1%type,
      mytable_col2 mytable.mytable_col2%type
);

type mytable_tab is table of mytable_rec;

l_mytable mytable_tab ;

--I've already loaded my query into l_mytable using bulk collect.
--Skipping it for readability

forall i in 1 .. l_mytable.count loop --The procedure won't compile
                                      --because of this line
      update mytable set
             mytable.mytable_col1 = i.mytable_col1,
             mytable.mytable_col2 = i.mytable_col2
       where 1 = 1; --some condition goes here
end loop;

forall i in l_mytable.first .. l_mytable.last loop --The procedure won't compile
                                                   --because of this line
      update mytable set
             mytable.mytable_col1 = i.mytable_col1,
             mytable.mytable_col2 = i.mytable_col2
       where 1 = 1; --some condition goes here
end loop;