Cross-module reference resolution error from SV wait statement - system-verilog

Question

I wanted to wait on the output variable of a task.
eg: wait(user_defined_task_name(output_variable_type_name) == 1)
In this example shown below, my intention is to make wait statement to be active from 0ns to 3ns (basically from the beginning of timestamp till t2=1)
Here is a working example;
class cl;
task run(output bit t);
$display("time=%0t , t=%0b",$realtime, t);
#1;
$display("time=%0t , t=%0b",$realtime, t);
#2;
t = 1;
$display("time=%0t , t=%0b",$realtime, t);
endtask
endclass
class c2 extends cl;
bit t2;
task run1();
wait(run(t2) == 1); // error from this line, what am i violating here?
$display("t2 working t2=%0b time = %0t", t2, $realtime);
endtask
endclass
module tmp;
c2 c2_h=new;
initial begin
c2_h.run1();
$display("test msg");
end
endmodule
eda output log:
Top Level Modules:
tmp TimeScale is 1 ns / 1 ns
Error-[XMREF] Cross-module reference resolution error testbench.sv, 19
Cross-module reference resolution error is found. Function is
expected, but actual target is not a function. Source info:
run(this.t2)
1 error CPU time: .116 seconds to compile Exit code expected: 0,
received: 1

Answer 1

A couple of problems with your code. A task does not return a value and cannot be used in an expression. You can only call it as a stand alone statement. But if you change run to a function functions cannot consume time.
In your particular example, you do not change the value of the t argument until the end of the task, and output arguments are copied out upon exiting the task, so you might as well just call the task run(t2) as a statement and it will block until returning.
task run1();
run(t2);
$display("t2 working t2=%0b time = %0t", t2, $realtime);
endtask
If on the otherhand run set the t argument somewhere in the middle of the task, and you want to continue the run1 task as soon as that happened, the you would have use a fork/join_none and a ref argument instead.
class cl;
task run(ref bit t);
$display("time=%0t , t=%0b",$realtime, t);
#1;
$display("time=%0t , t=%0b",$realtime, t);
#2;
t = 1;
#2;
$display("time=%0t , t=%0b",$realtime, t);
endtask
endclass
class c2 extends cl;
bit t2;
task run1();
fork
run(t2);
join_none
wait(t2 == 1);
$display("t2 working t2=%0b time = %0t", t2, $realtime);
endtask
endclass

Answer 2

When I run your code on the Cadence simulator, I get a different message:
xmvlog: *E,INVCTX The task 'run' cannot be used in this context.
Using the nchelp utility to get more information on that message:
A task or void function cannot be passed as an actual argument
because they do not return a value that can be used. They also cannot
be used as part of an expression.
In your simple example, there seems to be no need to use wait. You can simply call the task on its own:
task run1();
run(t2);
$display("t2 working t2=%0b time = %0t", t2, $realtime);
endtask