Collect classical registers using a set (#10496) (#10505)

* Collect classical registers using a set (#10496)

This adds a test case that adds multiple conditions on the same
register. The test case is fixed by using a set to collect the
classical registers in collapse_to_operation.

* Add release note

---------

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
(cherry picked from commit b47c28b)

qiskit/dagcircuit/collect_blocks.py

@@ -346,7 +346,7 @@ def collapse_to_operation(self, blocks, collapse_fn):
 
             # Additionally, find the set of classical registers used in conditions over full registers
             # (in such a case, we need to add that register to the block circuit, not just its clbits).
-            cur_clregs = []
+            cur_clregs = set()
 
             for node in block:
                 cur_qubits.update(node.qargs)
@@ -355,7 +355,7 @@ def collapse_to_operation(self, blocks, collapse_fn):
                 if cond is not None:
                     cur_clbits.update(condition_resources(cond).clbits)
                     if isinstance(cond[0], ClassicalRegister):
-                        cur_clregs.append(cond[0])
+                        cur_clregs.add(cond[0])
 
             # For reproducibility, order these qubits/clbits compatibly with the global order.
             sorted_qubits = sorted(cur_qubits, key=lambda x: global_index_map[x])

releasenotes/notes/fix-block-collapser-cregs-6c2d2dc6931d7bad.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The :class:`.BlockCollapser` transpiler pass will now correctly handle circuits that contain
+    more than one condition on the same classical register.

test/python/dagcircuit/test_collect_blocks.py

@@ -914,6 +914,23 @@ def test_split_layers_dagdependency(self):
         self.assertEqual(len(blocks[2]), 1)
         self.assertEqual(len(blocks[3]), 1)
 
+    def test_block_collapser_register_condition(self):
+        """Test that BlockCollapser can handle a register being used more than once."""
+        qc = QuantumCircuit(1, 2)
+        qc.x(0).c_if(qc.cregs[0], 0)
+        qc.y(0).c_if(qc.cregs[0], 1)
+
+        dag = circuit_to_dag(qc)
+        blocks = BlockCollector(dag).collect_all_matching_blocks(
+            lambda _: True, split_blocks=False, min_block_size=1
+        )
+        dag = BlockCollapser(dag).collapse_to_operation(blocks, lambda circ: circ.to_instruction())
+        collapsed_qc = dag_to_circuit(dag)
+
+        self.assertEqual(len(collapsed_qc.data), 1)
+        self.assertEqual(collapsed_qc.data[0].operation.definition.num_qubits, 1)
+        self.assertEqual(collapsed_qc.data[0].operation.definition.num_clbits, 2)
+
 
 if __name__ == "__main__":
     unittest.main()