Transpilation incorrectly removes necessary classical conditions

[p51lee]

Hello, I'm working with the Staq optimizer and it erroneously removes necessary if conditions.

To reproduce this issue, optimize the following bug.qasm:

❯ cat bug.qasm
OPENQASM 2.0;
include "qelib1.inc";
gate cx_o0 q0,q1 { x q0; cx q0,q1; x q0; }
qreg q[2];
creg c[1];
cx_o0 q[1],q[0];
swap q[0],q[1];
if(c==1) cx q[1],q[0];
measure q[0] -> c[0];

Since there are no prior measurements to change the initial state of c, the condition c==1 should be false and the cx gate should not be executed.

❯ staq --version
staq version 3.3
(c) 2019 - 2023 softwareQ Inc. All rights reserved.
❯ staq -O3 bug.qasm
OPENQASM 2.0;
include "qelib1.inc";

qreg q[2];
creg c[1];
x q[1];
cx q[1],q[0];
x q[1];
swap q[0],q[1];
cx q[1],q[0];
measure q[0] -> c[0];

However, Staq optimizer removes the condition if (c==1). It should not be removed because:

1. cx_o0 q[1],q[0]; flips q[0] to 1, because cx_o0 is a controlled-x gate with condition control qubit == 0.
2. By swap q[0],q[1]; now q[1] is 1.
3. if there's no if (c==1), cx q[1],q[0]; will erroneously flip q[0].

[meamy]

I tracked this down, it was a bug with one of the visitor classes from qasmtools. Fixed now. Thanks for the bug report!