Fix build filter coupling map with mix ideal/physical targets (backport #11009)

qiskit/transpiler/target.py

@@ -1073,7 +1073,7 @@ def build_coupling_map(self, two_q_gate=None, filter_idle_qubits=False):
             return None
 
     def _filter_coupling_graph(self):
-        has_operations = set(itertools.chain.from_iterable(self.qargs))
+        has_operations = set(itertools.chain.from_iterable(x for x in self.qargs if x is not None))
         graph = self._coupling_graph.copy()
         to_remove = set(graph.node_indices()).difference(has_operations)
         if to_remove:

releasenotes/notes/fix-handling-of-mixed-ideal-target-with-filter-qubits-171894cb758356ca.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed an issue in the :meth:`.Target.build_coupling_map` method when the
+    ``filter_idle_qubits`` argument was set to ``True`` and there was a mix
+    of fixed width ideal and physical instructions in the target. In these cases
+    previously the :meth:`.Target.build_coupling_map` would have raised an
+    exception because it was assuming all instructions in the target were
+    physical and defined over qubits.

test/python/transpiler/test_target.py

@@ -1837,6 +1837,23 @@ def test_coupling_map(self):
             set(self.ibm_target.build_coupling_map().get_edges()),
         )
 
+    def test_mixed_ideal_target_filtered_coupling_map(self):
+        target = Target(num_qubits=10)
+        target.add_instruction(
+            XGate(), {(qubit,): InstructionProperties(error=0.5) for qubit in range(5)}
+        )
+        target.add_instruction(
+            CXGate(),
+            {
+                edge: InstructionProperties(error=0.6)
+                for edge in CouplingMap.from_line(5, bidirectional=False).get_edges()
+            },
+        )
+        target.add_instruction(SXGate())
+        coupling_map = target.build_coupling_map(filter_idle_qubits=True)
+        self.assertEqual(max(coupling_map.physical_qubits), 4)
+        self.assertEqual(coupling_map.get_edges(), [(0, 1), (1, 2), (2, 3), (3, 4)])
+
 
 class TestInstructionProperties(QiskitTestCase):
     def test_empty_repr(self):