circuit_to_qasm_str proposed enhancement

[IlanIwumbwe]

Benny and I found this by running a randomly generated circuit through the pytket circuit_to_qasm_str function.

When we run this circuit:

Z_op = Op.create(OpType.Z)
H_op = Op.create(OpType.H)
U2_op = Op.create(OpType.U2, [-0.64, 0.86])
H_op = Op.create(OpType.H)
op_map = {(0,0,):Z_op,(0,1,):H_op,(1,0,):U2_op,(1,1,):H_op}
multiplexor = MultiplexorBox(op_map)

main_circ = Circuit(0, 2, "main_circ")

qreg_0 = main_circ.add_q_register("qreg_0",3)
qbit_3 = Qubit("qbit_3", 0)
main_circ.add_qubit(qbit_3)

creg_0 = main_circ.add_c_register("creg_0",4)

main_circ.add_gate(multiplexor,[qreg_0[0], qreg_0[1], qbit_3])

We get:

raise QASMUnsupportedError(
pytket.qasm.qasm.QASMUnsupportedError: Cannot print command of type: CnZ

A user might be confused by this if they didn't explicitly add CnZ gates into their circuit. After tracking down the root cause we saw that CnZ and CnY gates aren't supported here. These gates come from using Z or Y gates inside a multiplexor with more than 2 states. We feel that an Error message to that effect, perhaps here, would allow the user to debug this much faster.