but if I remove the keyword packed as you suggested, vcs reports 2 errors: Error-[IBC] Invalid bounds count sc_assignment_pattern_030.sv,29 The following access has an invalid number of indices.st4.st2.push_back Error-[XMRE] Cross Module reference resolution error sc_assignment_pattern_030.sv, 29 Error found while trying to resolve cross-module reference. token 'st2'. Originating module 'test'. Source Info:st4.st2.push_back((i+1)); then how should I fix these 2 errors?

I apologize for the mistake in my previous answer. The use of an unpacked array in the struct declaration requires an explicit size. To fix the syntax error and the cross-module reference error, you can modify the declaration of `st2` to include a size for the unpacked array.

Here is the corrected code:

module test;
  typedef struct packed {
    logic [9:-1][2:0] a;
  } ST1;

  typedef struct {
    ST1 st1;
    ST1 st2[$];
    logic [7:0] b;
  } ST4;

  initial begin
    ST4 st4;
    st4.st1.a[0][0] = 1;
    for (int i = 0; i < 5; i++) begin
      st4.st2.push_back(ST1'(i + 1));
    end
    st4.b = 8'hAA;
    $display("%s", st4.st2); //CHECK-001:'{a:'h1}, st2:'{'a:'h1}, {'a:'h2},{'a:'h3},{'a:'h4},{'a:'h5}}}
  end

endmodule

In this code, the declaration of `st2` now includes the size `[$]` to indicate that it is an unpacked array with a dynamic size. The `$` size specifier allows the size of the array to be determined at runtime, based on the number of elements that are pushed onto the array using the `push_back` method.

With this modification, the code should now compile without any syntax errors or cross-module reference errors.