Warning message while design compile

arg_copy is part of the libary:

  -- a_ext is the Altera version of the EXT function.  It is used to both
  -- zero-extend a signal to a new length, and to extract a signal of 'size'
  -- length from a larger signal.
  FUNCTION a_ext (arg : STD_LOGIC_VECTOR; size : INTEGER) RETURN STD_LOGIC_VECTOR IS
    VARIABLE arg_copy : STD_LOGIC_VECTOR ((arg'length - 1)DOWNTO 0) :=  arg ;
    VARIABLE result : STD_LOGIC_VECTOR((size-1) DOWNTO 0) := (others => '0');
    VARIABLE i : integer := 0;  
    VARIABLE bits_to_copy : integer := 0;
    VARIABLE arg_length : integer := arg'length ;
    VARIABLE LSB_bit : integer := 0;
  BEGIN 
    bits_to_copy := a_min(arg_length, size);
    FOR i IN 0 TO (bits_to_copy - 1) LOOP 
      result(i) := arg_copy(i);
    end LOOP;
    
    RETURN(result);   
  END;