Use of inout in task using fork-join_none

Question

Below two different coding style gives different results while using inout in automatic task.

Style 1:

task automatic sum (input int  x, input int y, inout int z);
    #10;
    z = x + y ;
endtask

int out;

initial begin
    fork
        sum(1,2,out);
    join_none
    #15;
    $display("sum output is %0d",out);  // Printing the expected value of 3
end

Style 2 :

task automatic sum (input int  x, input int y, inout int z);
    fork
        begin
            #10;
            z = x + y ;
        end
    join_none
endtask

int out;

initial begin        
    sum(1,2,out);
    #15;
    $display("sum output is %0d",out);  // Printing the value of 0
end

Can someone please explain why "Style 2" gives the value of 0?

Answer 1

At time 0, you call the sum task, and the inout port is assigned the value 0 since it is an int (whose default value is 0). The task forks off a process, then returns immediately due to the join_none, still at time 0. So the value of z is passed to the out variable as 0.

out does not receive the value of z after a delay of 10 or 15.

Refer to IEEE Std 1800-2023 section 13.3 Tasks:

inout // copy in at beginning and out at end