Update Instruction.condition_bits for runtime classical expressions (…

…#11325) (#11328)

I didn't even know this property existed and it wasn't tested directly,
but the IBM provider uses it during its custom scheduling passes, so
until removal, it should be kept updated.

(cherry picked from commit 5bbda4a)

Co-authored-by: Jake Lishman <[email protected]>

qiskit/circuit/instruction.py

@@ -618,12 +618,11 @@ def repeat(self, n):
     @property
     def condition_bits(self) -> List[Clbit]:
         """Get Clbits in condition."""
+        from qiskit.circuit.controlflow import condition_resources  # pylint: disable=cyclic-import
+
         if self.condition is None:
             return []
-        if isinstance(self.condition[0], Clbit):
-            return [self.condition[0]]
-        else:  # ClassicalRegister
-            return list(self.condition[0])
+        return list(condition_resources(self.condition).clbits)
 
     @property
     def name(self):

releasenotes/notes/fix-instruction-condition-bits-17694f98628b30ad.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The property :attr:`.Instruction.condition_bits` will now correctly handle runtime classical
+    expressions (:mod:`qiskit.circuit.classical`).

test/python/circuit/test_instructions.py

@@ -18,16 +18,20 @@
 
 import numpy as np
 
-from qiskit.circuit import Gate
-from qiskit.circuit import Parameter
-from qiskit.circuit import Instruction, InstructionSet
-from qiskit.circuit import QuantumCircuit
-from qiskit.circuit import QuantumRegister, ClassicalRegister, Qubit, Clbit
-from qiskit.circuit.library.standard_gates.h import HGate
-from qiskit.circuit.library.standard_gates.rz import RZGate
-from qiskit.circuit.library.standard_gates.x import CXGate
-from qiskit.circuit.library.standard_gates.s import SGate
-from qiskit.circuit.library.standard_gates.t import TGate
+from qiskit.circuit import (
+    Gate,
+    Parameter,
+    Instruction,
+    InstructionSet,
+    QuantumCircuit,
+    QuantumRegister,
+    ClassicalRegister,
+    Qubit,
+    Clbit,
+    IfElseOp,
+)
+from qiskit.circuit.library import HGate, RZGate, CXGate, SGate, TGate
+from qiskit.circuit.classical import expr
 from qiskit.test import QiskitTestCase
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.circuit.random import random_circuit
@@ -426,6 +430,26 @@ def test_repr_of_instructions(self):
             ),
         )
 
+    def test_instruction_condition_bits(self):
+        """Test that the ``condition_bits`` property behaves correctly until it is deprecated and
+        removed."""
+        bits = [Clbit(), Clbit()]
+        cr1 = ClassicalRegister(2, "cr1")
+        cr2 = ClassicalRegister(2, "cr2")
+        body = QuantumCircuit(cr1, cr2, bits)
+
+        def key(bit):
+            return body.find_bit(bit).index
+
+        op = IfElseOp((bits[0], False), body)
+        self.assertEqual(op.condition_bits, [bits[0]])
+
+        op = IfElseOp((cr1, 3), body)
+        self.assertEqual(op.condition_bits, list(cr1))
+
+        op = IfElseOp(expr.logic_and(bits[1], expr.equal(cr2, 3)), body)
+        self.assertEqual(sorted(op.condition_bits, key=key), sorted([bits[1]] + list(cr2), key=key))
+
     def test_instructionset_c_if_direct_resource(self):
         """Test that using :meth:`.InstructionSet.c_if` with an exact classical resource always
         works, and produces the expected condition."""