Fix transpile() with a Target containing an ideal Measure

This commit fixes an oversight in the transpile() function when running
with a Target (either directly or via a backend) that contains a
Measurement operation that is ideal (either globally or locally) with no
properties defined. This was not handled correctly in the function used
to convert a Target to a BackendProperties payload for passes that are
not yet target aware and this would cause an exception to be raised.
This commit fixes this edge case and excludes readout properties from
the generated BackendProperties in this case.

Fixes Qiskit#8969

qiskit/transpiler/target.py

@@ -1043,24 +1043,25 @@ def target_to_backend_properties(target: Target):
                     continue
                 qubit = qargs[0]
                 props_list = []
-                if props.error is not None:
-                    props_list.append(
-                        {
-                            "date": datetime.datetime.utcnow(),
-                            "name": "readout_error",
-                            "unit": "",
-                            "value": props.error,
-                        }
-                    )
-                if props.duration is not None:
-                    props_list.append(
-                        {
-                            "date": datetime.datetime.utcnow(),
-                            "name": "readout_length",
-                            "unit": "s",
-                            "value": props.duration,
-                        }
-                    )
+                if props is not None:
+                    if props.error is not None:
+                        props_list.append(
+                            {
+                                "date": datetime.datetime.utcnow(),
+                                "name": "readout_error",
+                                "unit": "",
+                                "value": props.error,
+                            }
+                        )
+                    if props.duration is not None:
+                        props_list.append(
+                            {
+                                "date": datetime.datetime.utcnow(),
+                                "name": "readout_length",
+                                "unit": "s",
+                                "value": props.duration,
+                            }
+                        )
                 if not props_list:
                     qubit_props = {}
                     break

releasenotes/notes/fix-transpile-ideal-measurement-c37e04533e196ded.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed an issue with :func:`~.transpile` when targeting a :class:`~.Target`
+    (either directly via the ``target`` argument or via a
+    :class:`~.BackendV2` instance from the ``backend`` argument) that
+    contained an ideal :class:`~.Measure` instruction (one that does not have
+    any properties defined). Previously this would raise an exception
+    trying to parse the target.
+    Fixed `#8969 <https://github.com/Qiskit/qiskit-terra/issues/8969>`__

test/python/compiler/test_transpiler.py

@@ -1726,6 +1726,19 @@ def test_qasm3_output_control_flow(self, optimization_level):
         # itself doesn't throw an error, though.
         self.assertIsInstance(qasm3.dumps(transpiled).strip(), str)
 
+    @data(0, 1, 2, 3)
+    def test_transpile_target_no_measurement_error(self, opt_level):
+        """Test that transpile with a target which contains ideal measurement works
+
+        Reproduce from https://github.com/Qiskit/qiskit-terra/issues/8969
+        """
+        target = Target()
+        target.add_instruction(Measure(), {(0,): None})
+        qc = QuantumCircuit(1, 1)
+        qc.measure(0, 0)
+        res = transpile(qc, target=target, optimization_level=opt_level)
+        self.assertEqual(qc, res)
+
 
 class StreamHandlerRaiseException(StreamHandler):
     """Handler class that will raise an exception on formatting errors."""