From 54f13cc7add0f8754a864b5519573b8179c4a3b4 Mon Sep 17 00:00:00 2001
From: "mergify[bot]" <37929162+mergify[bot]@users.noreply.github.com>
Date: Mon, 10 Jun 2024 14:19:50 +0200
Subject: [PATCH] Fix bugs with VF2Layout pass and Qiskit Aer 0.13 (#11585)
 (#12537)
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* Fix bugs with V2FLayout pass and Qiskit Aer 0.13

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* test

* Update releasenotes/notes/fix-vf2-aer-a7306ce07ea81700.yaml

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* long lines

---------

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>
(cherry picked from commit 2b847b8e9b8ca21e6c0e1ce26acb0686aec3175a)

Co-authored-by: Christopher J. Wood <cjwood@us.ibm.com>
---
 qiskit/transpiler/passes/layout/vf2_layout.py | 14 +++++++++----
 qiskit/transpiler/passes/layout/vf2_utils.py  |  4 ++--
 .../notes/fix-vf2-aer-a7306ce07ea81700.yaml   |  4 ++++
 test/python/transpiler/test_vf2_layout.py     | 20 +++++++++++++++++++
 4 files changed, 36 insertions(+), 6 deletions(-)
 create mode 100644 releasenotes/notes/fix-vf2-aer-a7306ce07ea81700.yaml

diff --git a/qiskit/transpiler/passes/layout/vf2_layout.py b/qiskit/transpiler/passes/layout/vf2_layout.py
index 4e3077eb1d4d..626f8f2b0fa3 100644
--- a/qiskit/transpiler/passes/layout/vf2_layout.py
+++ b/qiskit/transpiler/passes/layout/vf2_layout.py
@@ -104,15 +104,21 @@ def __init__(
                 limit on the number of trials will be set.
             target (Target): A target representing the backend device to run ``VF2Layout`` on.
                 If specified it will supersede a set value for ``properties`` and
-                ``coupling_map``.
+                ``coupling_map`` if the :class:`.Target` contains connectivity constraints. If the value
+                of ``target`` models an ideal backend without any constraints then the value of
+                ``coupling_map``
+                will be used.
 
         Raises:
             TypeError: At runtime, if neither ``coupling_map`` or ``target`` are provided.
         """
         super().__init__()
         self.target = target
-        if target is not None:
-            self.coupling_map = self.target.build_coupling_map()
+        if (
+            target is not None
+            and (target_coupling_map := self.target.build_coupling_map()) is not None
+        ):
+            self.coupling_map = target_coupling_map
         else:
             self.coupling_map = coupling_map
         self.properties = properties
@@ -145,7 +151,7 @@ def run(self, dag):
         )
         # Filter qubits without any supported operations. If they don't support any operations
         # They're not valid for layout selection
-        if self.target is not None:
+        if self.target is not None and self.target.qargs is not None:
             has_operations = set(itertools.chain.from_iterable(self.target.qargs))
             to_remove = set(range(len(cm_nodes))).difference(has_operations)
             if to_remove:
diff --git a/qiskit/transpiler/passes/layout/vf2_utils.py b/qiskit/transpiler/passes/layout/vf2_utils.py
index 99006017482c..c5d420127f88 100644
--- a/qiskit/transpiler/passes/layout/vf2_utils.py
+++ b/qiskit/transpiler/passes/layout/vf2_utils.py
@@ -145,7 +145,7 @@ def score_layout(
 def build_average_error_map(target, properties, coupling_map):
     """Build an average error map used for scoring layouts pre-basis translation."""
     num_qubits = 0
-    if target is not None:
+    if target is not None and target.qargs is not None:
         num_qubits = target.num_qubits
         avg_map = ErrorMap(len(target.qargs))
     elif coupling_map is not None:
@@ -157,7 +157,7 @@ def build_average_error_map(target, properties, coupling_map):
         # object
         avg_map = ErrorMap(0)
     built = False
-    if target is not None:
+    if target is not None and target.qargs is not None:
         for qargs in target.qargs:
             if qargs is None:
                 continue
diff --git a/releasenotes/notes/fix-vf2-aer-a7306ce07ea81700.yaml b/releasenotes/notes/fix-vf2-aer-a7306ce07ea81700.yaml
new file mode 100644
index 000000000000..52ea96d0984b
--- /dev/null
+++ b/releasenotes/notes/fix-vf2-aer-a7306ce07ea81700.yaml
@@ -0,0 +1,4 @@
+fixes:
+  - |
+    The :class:`.VF2Layout` pass would raise an exception when provided with a :class:`.Target` instance without connectivity constraints.
+    This would be the case with targets from Aer 0.13. The issue is now fixed.
diff --git a/test/python/transpiler/test_vf2_layout.py b/test/python/transpiler/test_vf2_layout.py
index b0957c824688..716e49d35009 100644
--- a/test/python/transpiler/test_vf2_layout.py
+++ b/test/python/transpiler/test_vf2_layout.py
@@ -570,6 +570,26 @@ def test_3_q_gate(self):
             pass_1.property_set["VF2Layout_stop_reason"], VF2LayoutStopReason.MORE_THAN_2Q
         )
 
+    def test_target_without_coupling_map(self):
+        """When a target has no coupling_map but it is provided as argument.
+        See: https://github.com/Qiskit/qiskit/pull/11585"""
+
+        circuit = QuantumCircuit(3)
+        circuit.cx(0, 1)
+        dag = circuit_to_dag(circuit)
+
+        target = Target(num_qubits=3)
+        target.add_instruction(CXGate())
+
+        vf2_pass = VF2Layout(
+            coupling_map=CouplingMap([[0, 2], [1, 2]]), target=target, seed=42, max_trials=1
+        )
+        vf2_pass.run(dag)
+
+        self.assertEqual(
+            vf2_pass.property_set["VF2Layout_stop_reason"], VF2LayoutStopReason.SOLUTION_FOUND
+        )
+
 
 class TestMultipleTrials(QiskitTestCase):
     """Test the passes behavior with >1 trial."""