Fix Instruction.repeat with conditionals (Qiskit#11940)

* Fix `Instruction.repeat` with conditionals

We can't put register conditionals within an `Instruction.definition`
field; the data model of `QuantumCircuit` doesn't permit closing over
registers from within definitions.  This commit moves a condition to the
outer `Instruction` that's returned.

* Avoid 'to_mutable' if not needed

* Add proviso on repeated conditionals in documentation

* Update wording in release note

Co-authored-by: Luciano Bello <[email protected]>

---------

Co-authored-by: Luciano Bello <[email protected]>

qiskit/circuit/instruction.py

@@ -41,7 +41,6 @@
 import numpy
 
 from qiskit.circuit.exceptions import CircuitError
-from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit.classicalregister import ClassicalRegister, Clbit
 from qiskit.qobj.qasm_qobj import QasmQobjInstruction
 from qiskit.circuit.parameter import ParameterExpression
@@ -560,7 +559,13 @@ def _return_repeat(self, exponent):
         )
 
     def repeat(self, n):
-        """Creates an instruction with `gate` repeated `n` amount of times.
+        """Creates an instruction with ``self`` repeated :math`n` times.
+
+        If this operation has a conditional, the output instruction will have the same conditional
+        and the inner repeated operations will be unconditional; instructions within a compound
+        definition cannot be conditioned on registers within Qiskit's data model.  This means that
+        it is not valid to apply a repeated instruction to a clbit that it both writes to and reads
+        from in its condition.
 
         Args:
             n (int): Number of times to repeat the instruction
@@ -577,22 +582,24 @@ def repeat(self, n):
         n = int(n)
 
         instruction = self._return_repeat(n)
-        qargs = [] if self.num_qubits == 0 else QuantumRegister(self.num_qubits, "q")
-        cargs = [] if self.num_clbits == 0 else ClassicalRegister(self.num_clbits, "c")
-
         if instruction.definition is None:
             # pylint: disable=cyclic-import
             from qiskit.circuit import QuantumCircuit, CircuitInstruction
 
-            qc = QuantumCircuit()
-            if qargs:
-                qc.add_register(qargs)
-            if cargs:
-                qc.add_register(cargs)
-            circuit_instruction = CircuitInstruction(self, qargs, cargs)
+            qc = QuantumCircuit(self.num_qubits, self.num_clbits)
+            qargs = tuple(qc.qubits)
+            cargs = tuple(qc.clbits)
+            base = self.copy()
+            if self.condition:
+                # Condition is handled on the outer instruction.
+                base = base.to_mutable()
+                base.condition = None
             for _ in [None] * n:
-                qc._append(circuit_instruction)
-        instruction.definition = qc
+                qc._append(CircuitInstruction(base, qargs, cargs))
+
+            instruction.definition = qc
+        if self.condition:
+            instruction = instruction.c_if(*self.condition)
         return instruction
 
     @property

releasenotes/notes/fix-instruction-repeat-conditional-dfe4d7ced54a7bb6.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    The method :meth:`.Instruction.repeat` now moves a set :attr:`~.Instruction.condition` to the
+    outer returned :class:`~.circuit.Instruction` and leave the inner gates of its definition
+    unconditional.  Previously, the method would leave :class:`.ClassicalRegister` instances within
+    the inner definition, which was an invalid state, and would manifest itself as seemingly unrelated
+    bugs later, such as during transpilation or export.  Fixed `#11935 <https://github.com/Qiskit/qiskit/issues/11935>`__.

test/python/circuit/test_instruction_repeat.py

@@ -52,6 +52,18 @@ def test_standard_1Q_one(self):
         self.assertEqual(result.definition, expected.definition)
         self.assertIsInstance(result, Gate)
 
+    def test_conditional(self):
+        """Test that repetition works with a condition."""
+        cr = ClassicalRegister(3, "cr")
+        gate = SGate().c_if(cr, 7).repeat(5)
+        self.assertEqual(gate.condition, (cr, 7))
+
+        defn = QuantumCircuit(1)
+        for _ in range(5):
+            # No conditions on the inner bit.
+            defn.s(0)
+        self.assertEqual(gate.definition, defn)
+
 
 class TestRepeatInt2Q(QiskitTestCase):
     """Test gate_q2.repeat() with integer"""
@@ -83,6 +95,18 @@ def test_standard_2Q_one(self):
         self.assertEqual(result.definition, expected.definition)
         self.assertIsInstance(result, Gate)
 
+    def test_conditional(self):
+        """Test that repetition works with a condition."""
+        cr = ClassicalRegister(3, "cr")
+        gate = CXGate().c_if(cr, 7).repeat(5)
+        self.assertEqual(gate.condition, (cr, 7))
+
+        defn = QuantumCircuit(2)
+        for _ in range(5):
+            # No conditions on the inner bit.
+            defn.cx(0, 1)
+        self.assertEqual(gate.definition, defn)
+
 
 class TestRepeatIntMeasure(QiskitTestCase):
     """Test Measure.repeat() with integer"""
@@ -118,6 +142,18 @@ def test_measure_one(self):
         self.assertIsInstance(result, Instruction)
         self.assertNotIsInstance(result, Gate)
 
+    def test_measure_conditional(self):
+        """Test conditional measure moves condition to the outside."""
+        cr = ClassicalRegister(3, "cr")
+        measure = Measure().c_if(cr, 7).repeat(5)
+        self.assertEqual(measure.condition, (cr, 7))
+
+        defn = QuantumCircuit(1, 1)
+        for _ in range(5):
+            # No conditions on the inner bit.
+            defn.measure(0, 0)
+        self.assertEqual(measure.definition, defn)
+
 
 class TestRepeatErrors(QiskitTestCase):
     """Test when Gate.repeat() should raise."""