Fix `StatevectorSampler` to raise an error if a circuit with c_if is passed (#12842)

* fix StatevectorSampler with c_if

* add comment

qiskit/primitives/statevector_sampler.py

@@ -207,7 +207,7 @@ def _preprocess_circuit(circuit: QuantumCircuit):
     qargs_index = {v: k for k, v in enumerate(qargs)}
     circuit = circuit.remove_final_measurements(inplace=False)
     if _has_control_flow(circuit):
-        raise QiskitError("StatevectorSampler cannot handle ControlFlowOp")
+        raise QiskitError("StatevectorSampler cannot handle ControlFlowOp and c_if")
     if _has_measure(circuit):
         raise QiskitError("StatevectorSampler cannot handle mid-circuit measurements")
     # num_qubits is used as sentinel to fill 0 in _samples_to_packed_array
@@ -283,4 +283,7 @@ def _final_measurement_mapping(circuit: QuantumCircuit) -> dict[tuple[ClassicalR
 
 
 def _has_control_flow(circuit: QuantumCircuit) -> bool:
-    return any(isinstance(instruction.operation, ControlFlowOp) for instruction in circuit)
+    return any(
+        isinstance((op := instruction.operation), ControlFlowOp) or op.condition
+        for instruction in circuit
+    )

releasenotes/notes/fix-statevector-sampler-c_if-9753f8f97a3d0ff5.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug of :class:`.StatevectorSampler` that ignored gates with ``c_if``.
+    It will raise an error because :class:`.Statevector` cannot handle ``c_if``.

test/python/primitives/test_statevector_sampler.py

@@ -280,6 +280,11 @@ class TestStatevectorSampler(QiskitTestCase):
         qc3 = QuantumCircuit(1, 1)
         with qc3.for_loop(range(5)):
             qc3.h(0)
+        qc4 = QuantumCircuit(2, 2)
+        qc4.h(0)
+        qc4.measure(1, 1)
+        qc4.x(0).c_if(1, 1)
+        qc4.measure(0, 0)
 
         sampler = StatevectorSampler()
         with self.subTest("set parameter values to a non-parameterized circuit"):
@@ -301,6 +306,9 @@ class TestStatevectorSampler(QiskitTestCase):
         with self.subTest("with control flow"):
             with self.assertRaises(QiskitError):
                 _ = sampler.run([qc3]).result()
+        with self.subTest("with c_if"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc4]).result()
         with self.subTest("negative shots, run arg"):
             with self.assertRaises(ValueError):
                 _ = sampler.run([qc1], shots=-1).result()
@@ -584,17 +592,21 @@ class TestStatevectorSampler(QiskitTestCase):
         c2 = ClassicalRegister(1, "c2")
 
         qc = QuantumCircuit(q, c1, c2)
-        qc.ry(np.pi / 4, 2)
-        qc.cx(2, 1)
-        qc.cx(0, 1)
-        qc.h(0)
-        qc.measure(0, c1)
-        qc.measure(1, c2)
         qc.z(2).c_if(c1, 1)
         qc.x(2).c_if(c2, 1)
         qc2 = QuantumCircuit(5, 5)
         qc2.compose(qc, [0, 2, 3], [2, 4], inplace=True)
-        cregs = [creg.name for creg in qc2.cregs]
+        # Note: qc2 has aliased cregs, c0 -> c[2] and c1 -> c[4].
+        # copy_empty_like copies the aliased cregs of qc2 to qc3.
+        qc3 = QuantumCircuit.copy_empty_like(qc2)
+        qc3.ry(np.pi / 4, 2)
+        qc3.cx(2, 1)
+        qc3.cx(0, 1)
+        qc3.h(0)
+        qc3.measure(0, 2)
+        qc3.measure(1, 4)
+        self.assertEqual(len(qc3.cregs), 3)
+        cregs = [creg.name for creg in qc3.cregs]
         target = {
             cregs[0]: {0: 4255, 4: 4297, 16: 720, 20: 726},
             cregs[1]: {0: 5000, 1: 5000},
@@ -602,7 +614,7 @@ class TestStatevectorSampler(QiskitTestCase):
         }
 
         sampler = StatevectorSampler(seed=self._seed)
-        result = sampler.run([qc2], shots=self._shots).result()
+        result = sampler.run([qc3], shots=self._shots).result()
         self.assertEqual(len(result), 1)
         data = result[0].data
         self.assertEqual(len(data), 3)