relax a check of measurements for Sampler

qiskit/primitives/base_sampler.py

@@ -105,7 +105,6 @@
 from qiskit.utils.deprecation import deprecate_arguments, deprecate_function
 
 from .sampler_result import SamplerResult
-from .utils import final_measurement_mapping
 
 
 class BaseSampler(ABC):
@@ -393,14 +392,6 @@ def run(
                     "on the desired qubits."
                 )
 
-            mapping = final_measurement_mapping(circuit)
-            if set(range(circuit.num_clbits)) != set(mapping.values()):
-                raise QiskitError(
-                    f"Some classical bits of the {i}-th circuit are not used for measurements."
-                    f" the number of classical bits ({circuit.num_clbits}),"
-                    f" the used classical bits ({set(mapping.values())})."
-                )
-
         run_opts = copy(self.run_options)
         run_opts.update_options(**run_options)
 

qiskit/primitives/sampler.py

@@ -158,6 +158,12 @@ def _run(
     def _preprocess_circuit(circuit: QuantumCircuit):
         circuit = init_circuit(circuit)
         q_c_mapping = final_measurement_mapping(circuit)
+        if set(range(circuit.num_clbits)) != set(q_c_mapping.values()):
+            raise QiskitError(
+                "Some classical bits are not used for measurements."
+                f" the number of classical bits ({circuit.num_clbits}),"
+                f" the used classical bits ({set(q_c_mapping.values())})."
+            )
         c_q_mapping = sorted((c, q) for q, c in q_c_mapping.items())
         qargs = [q for _, q in c_q_mapping]
         circuit = circuit.remove_final_measurements(inplace=False)

releasenotes/notes/add-sampler-error-check-38426fb186db44d4.yaml

@@ -1,7 +1,6 @@
 ---
 upgrade:
   - |
-    Added some error checks to :meth:`~qiskit.primitives.BaseSampler.run`.
-    It raises an error if there is no classical bit or some classical bits
-    are not used for measurements.
+    Added a validation check to :meth:`~qiskit.primitives.BaseSampler.run`.
+    It raises an error if there is no classical bit.