randomize() question

Hi all,

There is a randomize() problem.
I want to randomize a Packet_seq which includes two Packet object.

First, I call Packet_seq randomize() to get pkt_seq. From pkt_seq to get Packet's cmd by constraint.
Second, I call Packet randomize() from Packet_seq's post_randomize() to make Packet's other attribute randomized.

But the random solver said there is a error when solve my constraint?
Any suggestions are welcome!


// Code listed below (and is attached)
module top;

typedef enum { WRITE=1,READ=2,IDLE=4} cmd_t;
typedef enum { WRITE_IDLE, READ_IDLE} packet_seq_t;
   
    class Packet;
       
        rand bit [7:0] addr;
        rand bit [31:0] data;
        cmd_t cmd;
        rand bit [2:0] dly; 
        
        function new (cmd_t cmd_new = IDLE,
                      bit [31:0] data_new = 0,
                      bit [7:0] addr_new = 0,
                      bit [2:0] dly_new = 1);
            cmd = cmd_new;
            data = data_new;
            addr = addr_new;
            dly = dly_new;
        endfunction

        function void do_print ();
        begin
            $display("Packet_tr: cmd=%s, addr=%0d, data=%0d, dly=%0d", cmd.name(), addr, data, dly);
        end
        endfunction
   endclass : Packet

   class Packet_seq;
       rand packet_seq_t pkt_seq;
       Packet pkt[2];
      
       constraint pkt_seq_1_cst { pkt_seq == WRITE_IDLE -> (pkt[0].cmd == WRITE && pkt[1].cmd == IDLE); }
       constraint pkt_seq_2_cst { pkt_seq == READ_IDLE  -> (pkt[0].cmd == READ  && pkt[1].cmd == IDLE); }
      
       function new ();
           for (int i=0;i
               pkt[i] = new();
           end
       endfunction

       function void post_randomize();
           int random;
           for (int i=0;i
               random = pkt[i].randomize();
           end
       endfunction
      
       function void do_print ();
           $display ("Packet Sequence is %s", pkt_seq.name());
           for (int i=0;i
               pkt[i].do_print();
           end
       endfunction
  
   endclass : Packet_seq

  //------------------
  // Main routain 
  //------------------
  Packet_seq pkt_seq[3];
   initial begin
       int random;
       for(int i=0;i
           pkt_seq[i] = new();
           random = pkt_seq[i].randomize();
           pkt_seq[i].do_print();
       end
   end
  
 
endmodule