[sv-bc] packed struct with 2 and 4 val fields

struct packed {reg[1:0][1:0] r; bit [1:0][1:0]b;} s;
module top;
     initial begin
         s.b='x;
         $display("%b", s); //should print all x or xxxx0000???
     end
endmodule

In above example s is 4 value as stated in LRM
My question is what should be written into s.b when it is assigned with 'x.
LRM says that when writing to a 2-val field then implicit 2val to 4 val
cast occurs.
But in this case I'm writing 4 value value into 2 val field.
In my opinion this LRM statement about 2val to 4val implicit cast is not
clear.

DANiel

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