Code Bug with Synthesizing Permutation Matrix #4928

def mod_mult_gate(b, N):
    """
    Modular multiplication gate from permutation matrix.
    """
    if gcd(b, N) > 1:
        print(f"Error: gcd({b},{N}) > 1")
    else:
        n = floor(log(N - 1, 2)) + 1
        U = np.full((2**n, 2**n), 0)
        for x in range(N):
            U[b * x % N][x] = 1
        for x in range(N, 2**n):
            U[x][x] = 1
        G = UnitaryGate(U)
        G.name = f"M_{b}"
        return G

# Let's build M2 using the permutation matrix definition
M2_other = mod_mult_gate(2, 15)

# Add it to a circuit
circ = QuantumCircuit(4)
circ.compose(M2_other, inplace=True)
circ = circ.decompose()

# Transpile the circuit and get the depth
coupling_map = CouplingMap.from_line(4)
pm = generate_preset_pass_manager(coupling_map=coupling_map)
transpiled_circ = pm.run(circ)

print(f"qubits: {circ.num_qubits}")
print(
    f"2q-depth: {transpiled_circ.depth(lambda x: x.operation.num_qubits==2)}"
)
print(f"2q-size: {transpiled_circ.size(lambda x: x.operation.num_qubits==2)}")
print(f"Operator counts: {transpiled_circ.count_ops()}")
transpiled_circ.decompose().draw(
    output="mpl", fold=-1, style="clifford", idle_wires=False
)

qubits: 4
2q-depth: 94
2q-size: 96
Operator counts: OrderedDict({'cx': 45, 'swap': 32, 'u': 24, 'u1': 7, 'u3': 4, 'unitary': 3, 'circuit-335': 1, 'circuit-338': 1, 'circuit-341': 1, 'circuit-344': 1, 'circuit-347': 1, 'circuit-350': 1, 'circuit-353': 1, 'circuit-356': 1, 'circuit-359': 1, 'circuit-362': 1, 'circuit-365': 1, 'circuit-368': 1, 'circuit-371': 1, 'circuit-374': 1, 'circuit-377': 1, 'circuit-380': 1})

---------------------------------------------------------------------------
PanicException                            Traceback (most recent call last)
Cell In[24], line 7
      5 circ = QuantumCircuit(4)
      6 circ.compose(M2_other, inplace=True)
----> 7 circ = circ.decompose()
      9 # Transpile the circuit and get the depth
     10 coupling_map = CouplingMap.from_line(4)

File ~/.venv/lib/python3.13/site-packages/qiskit/circuit/quantumcircuit.py:3843, in QuantumCircuit.decompose(self, gates_to_decompose, reps)
   3841 pass_ = Decompose(gates_to_decompose, apply_synthesis=True)
   3842 for _ in range(reps):
-> 3843     dag = pass_.run(dag)
   3845 # do not copy operations, this is done in the conversion with circuit_to_dag
   3846 return dag_to_circuit(dag, copy_operations=False)

File ~/.venv/lib/python3.13/site-packages/qiskit/transpiler/passes/basis/decompose.py:74, in Decompose.run(self, dag)
     71     decomposition = control_flow.map_blocks(self.run, node.op)
     72     dag.substitute_node(node, decomposition, inplace=True)
---> 74 elif getattr(node.op, "definition", None) is None:
     75     # if we try to synthesize, turn the node into a DAGCircuit and run HLS
     76     if self.apply_synthesis:
     77         node_as_dag = _node_to_dag(node)

File ~/.venv/lib/python3.13/site-packages/qiskit/circuit/instruction.py:296, in Instruction.definition(self)
    294 """Return definition in terms of other basic gates."""
    295 if self._definition is None:
--> 296     self._define()
    297 return self._definition

File ~/.venv/lib/python3.13/site-packages/qiskit/circuit/library/generalized_gates/unitary.py:162, in UnitaryGate._define(self)
    157 else:
    158     from qiskit.synthesis.unitary.qsd import (  # pylint: disable=cyclic-import
    159         qs_decomposition,
    160     )
--> 162     self.definition = qs_decomposition(self.to_matrix())
    163     # Since iterative Quantum Shannon Decomposition may provide imprecise matrices,
    164     # we use the Isometry decomposition in this case
    165     # pylint: disable=cyclic-import
    166     from qiskit.quantum_info.operators import Operator

File ~/.venv/lib/python3.13/site-packages/qiskit/synthesis/unitary/qsd.py:122, in qs_decomposition(mat, opt_a1, opt_a2, decomposer_1q, decomposer_2q, _depth)
    119         two_q_decomp = decomposer_2q._inner_decomposer
    120     array = np.asarray(mat, dtype=complex)
    121     return QuantumCircuit._from_circuit_data(
--> 122         qsd.qs_decomposition(array, opt_a1, opt_a2, basis_1q, two_q_decomp)
    123     )
    125 #  _depth (int): Internal use parameter to track recursion depth.
    126 dim = mat.shape[0]

PanicException: called `Option::unwrap()` on a `None` value