This code seems to work as a better workaround for the problem. 

   leading_white_space: INTEGER
      local
         i, l_count: INTEGER; l_area: like area; c_i: CHARACTER
         l_prop: like character_properties
      do
         l_area := area; l_count := count; l_prop := character_properties
         from i := 0 until i = l_count loop
            c_i := l_area [i]
            -- `Unencoded_character' is space
            if c_i = Unencoded_character then
               if l_prop.is_space (unencoded_item (i + 1)) then
                  Result := Result + 1
               else
                  i := l_count - 1 -- break out of loop
               end
            elseif c_i.is_space then
               Result := Result + 1
            else
               i := l_count - 1 -- break out of loop
            end
            i := i + 1
         end
      ensure
         substring_agrees: across substring (1, Result) as char all character_properties.is_space (char.item) end
      end