Fix classical-synthesis label ordering with registerless=False (#9536)

* follow the Tweedledum convention on endianness

* reno

* add test

* optional dep

* docstring

* reno

* attempt to fix sphinx

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>

qiskit/circuit/classicalfunction/classicalfunction.py

@@ -168,7 +168,6 @@ def _define(self):
     def qregs(self):
         """The list of qregs used by the classicalfunction"""
         qregs = [QuantumRegister(1, name=arg) for arg in self.args if self.types[0][arg] == "Int1"]
-        qregs.reverse()
         if self.types[0]["return"] == "Int1":
             qregs.append(QuantumRegister(1, name="return"))
         return qregs

qiskit/circuit/classicalfunction/utils.py

@@ -75,8 +75,7 @@ def tweedledum2qiskit(tweedledum_circuit, name=None, qregs=None):
         QuantumCircuit: The Tweedledum circuit converted to a Qiskit circuit.
 
     Raises:
-        ClassicalFunctionCompilerError: If there a gate in the Tweedledum circuit has no Qiskit
-        equivalent.
+        ClassicalFunctionCompilerError: If a gate in the Tweedledum circuit has no Qiskit equivalent.
     """
     if qregs:
         qiskit_qc = QuantumCircuit(*qregs, name=name)

releasenotes/notes/fix_9363-445db8fde1244e57.yaml

@@ -0,0 +1,40 @@
+---
+fixes:
+  - |
+    Fixed `#9363 <https://github.com/Qiskit/qiskit-terra/issues/9363>`__.
+    by labeling the non-registerless synthesis in the order that Tweedledum
+    returns. For example, compare this example before and after the fix::
+
+      from qiskit.circuit import QuantumCircuit
+      from qiskit.circuit.classicalfunction import BooleanExpression
+
+      boolean_exp = BooleanExpression.from_dimacs_file("simple_v3_c2.cnf")
+      circuit = QuantumCircuit(boolean_exp.num_qubits)
+      circuit.append(boolean_exp, range(boolean_exp.num_qubits))
+      circuit.draw("text")
+
+      from qiskit.circuit.classicalfunction import classical_function
+      from qiskit.circuit.classicalfunction.types import Int1
+
+      @classical_function
+      def grover_oracle(a: Int1, b: Int1, c: Int1) -> Int1:
+          return (a and b and not c)
+
+      quantum_circuit = grover_oracle.synth(registerless=False)
+      print(quantum_circuit.draw())
+
+    Which would print
+
+    .. parsed-literal::
+
+           Before             After
+
+           c: ──■──           a: ──■──
+                │                  │
+           b: ──■──           b: ──■──
+                │                  │
+           a: ──o──           c: ──o──
+              ┌─┴─┐              ┌─┴─┐
+      return: ┤ X ├      return: ┤ X ├
+              └───┘              └───┘
+

test/python/classical_function_compiler/test_synthesis.py

@@ -49,10 +49,10 @@ def test_grover_oracle_arg_regs(self):
         qr_c = QuantumRegister(1, "c")
         qr_d = QuantumRegister(1, "d")
         qr_return = QuantumRegister(1, "return")
-        expected = QuantumCircuit(qr_d, qr_c, qr_b, qr_a, qr_return)
+        expected = QuantumCircuit(qr_a, qr_b, qr_c, qr_d, qr_return)
         expected.append(
             XGate().control(4, ctrl_state="1010"),
-            [qr_d[0], qr_c[0], qr_b[0], qr_a[0], qr_return[0]],
+            [qr_a[0], qr_b[0], qr_c[0], qr_d[0], qr_return[0]],
         )
 
         self.assertEqual(quantum_circuit.name, "grover_oracle")

test/python/visualization/test_circuit_text_drawer.py

@@ -48,8 +48,13 @@
     CPhaseGate,
 )
 from qiskit.transpiler.passes import ApplyLayout
+from qiskit.utils.optionals import HAS_TWEEDLEDUM
 from .visualization import path_to_diagram_reference, QiskitVisualizationTestCase
 
+if HAS_TWEEDLEDUM:
+    from qiskit.circuit.classicalfunction import classical_function
+    from qiskit.circuit.classicalfunction.types import Int1
+
 
 class TestTextDrawerElement(QiskitTestCase):
     """Draw each element"""
@@ -1216,6 +1221,31 @@ def test_text_spacing_2378(self):
         circuit.rz(11111, qr[2])
         self.assertEqual(str(_text_circuit_drawer(circuit)), expected)
 
+    @unittest.skipUnless(HAS_TWEEDLEDUM, "Tweedledum is required for these tests.")
+    def test_text_synth_no_registerless(self):
+        """Test synthesis's label when registerless=False.
+        See https://github.com/Qiskit/qiskit-terra/issues/9363"""
+        expected = "\n".join(
+            [
+                "                ",
+                "     a: |0>──■──",
+                "             │  ",
+                "     b: |0>──■──",
+                "             │  ",
+                "     c: |0>──o──",
+                "           ┌─┴─┐",
+                "return: |0>┤ X ├",
+                "           └───┘",
+            ]
+        )
+
+        @classical_function
+        def grover_oracle(a: Int1, b: Int1, c: Int1) -> Int1:
+            return a and b and not c
+
+        circuit = grover_oracle.synth(registerless=False)
+        self.assertEqual(str(_text_circuit_drawer(circuit)), expected)
+
 
 class TestTextDrawerLabels(QiskitTestCase):
     """Gates with labels."""