Fix use of node.op in Split2QUnitaries (backport #13015) (#13062)

* Fix  use of `node.op` in `Split2QUnitaries` (#13015)

* Replace node.op with node.matrix

* Add docstring to test gate

* Fix black

(cherry picked from commit 623415c)

* Update test/python/transpiler/test_split_2q_unitaries.py

---------

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>
Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/transpiler/passes/optimization/split_2q_unitaries.py

@@ -50,7 +50,7 @@ def run(self, dag: DAGCircuit):
             ):
                 continue
 
-            decomp = TwoQubitWeylDecomposition(node.op, fidelity=self.requested_fidelity)
+            decomp = TwoQubitWeylDecomposition(node.matrix, fidelity=self.requested_fidelity)
             if (
                 decomp._inner_decomposition.specialization
                 == TwoQubitWeylDecomposition._specializations.IdEquiv

releasenotes/notes/fix-split-2q-unitaries-custom-gate-d10f7670a35548f4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`.Split2QUnitaries` where it would fail to run on circuits
+    with custom gates that didn't implement :meth:`__array__`.
+    See `#12984 <https://github.com/Qiskit/qiskit/issues/12970>`__.

test/python/transpiler/test_split_2q_unitaries.py

@@ -19,7 +19,7 @@
 
 from qiskit import QuantumCircuit, QuantumRegister, transpile
 from qiskit.circuit.library import UnitaryGate, XGate, ZGate, HGate
-from qiskit.circuit import Parameter, CircuitInstruction
+from qiskit.circuit import Parameter, CircuitInstruction, Gate
 from qiskit.providers.fake_provider import GenericBackendV2
 from qiskit.quantum_info import Operator
 from qiskit.transpiler import PassManager
@@ -223,3 +223,39 @@ def test_split_qft(self):
         pm.append(Split2QUnitaries())
         qc_split = pm.run(qc)
         self.assertEqual(26, qc_split.num_nonlocal_gates())
+
+    def test_gate_no_array(self):
+        """
+        Test that the pass doesn't fail when the circuit contains a custom gate
+        with no ``__array__`` implementation.
+
+        Reproduce from: https://github.com/Qiskit/qiskit/issues/12970
+        """
+
+        class MyGate(Gate):
+            """Custom gate"""
+
+            def __init__(self):
+                super().__init__("mygate", 2, [])
+
+            def to_matrix(self):
+                return np.eye(4, dtype=complex)
+
+        def mygate(self, qubit1, qubit2):
+            return self.append(MyGate(), [qubit1, qubit2], [])
+
+        QuantumCircuit.mygate = mygate
+
+        qc = QuantumCircuit(2)
+        qc.mygate(0, 1)
+
+        pm = PassManager()
+        pm.append(Collect2qBlocks())
+        pm.append(ConsolidateBlocks())
+        pm.append(Split2QUnitaries())
+        qc_split = pm.run(qc)
+
+        self.assertTrue(Operator(qc).equiv(qc_split))
+        self.assertTrue(
+            matrix_equal(Operator(qc).data, Operator(qc_split).data, ignore_phase=False)
+        )