Add validation to Sampler if there are no measurements

qiskit/primitives/base/base_sampler.py

@@ -80,7 +80,7 @@
 from copy import copy
 from typing import Generic, TypeVar
 
-from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import ControlFlowOp, Measure, QuantumCircuit
 from qiskit.circuit.parametertable import ParameterView
 from qiskit.providers import JobV1 as Job
 
@@ -159,7 +159,6 @@ def _run(
     ) -> T:
         raise NotImplementedError("The subclass of BaseSampler must implment `_run` method.")
 
-    # TODO: validate measurement gates are present
     @classmethod
     def _validate_circuits(
         cls,
@@ -173,6 +172,11 @@ def _validate_circuits(
                     "Sampler requires classical bits, plus measurements "
                     "on the desired qubits."
                 )
+            if not _has_measure(circuit):
+                raise ValueError(
+                    f"The {i}-th circuit does not have Measure instruction. "
+                    "Without measurements, the circuit cannot be sampled from."
+                )
         return circuits
 
     @property
@@ -192,3 +196,14 @@ def parameters(self) -> tuple[ParameterView, ...]:
             List of the parameters in each quantum circuit.
         """
         return tuple(self._parameters)
+
+
+def _has_measure(circuit: QuantumCircuit) -> bool:
+    for instruction in circuit:
+        if isinstance(instruction.operation, Measure):
+            return True
+        elif isinstance(instruction.operation, ControlFlowOp):
+            for block in instruction.operation.blocks:
+                if _has_measure(block):
+                    return True
+    return False

releasenotes/notes/sampler-with-no-measure-6cd3a05555187b58.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Add early validation for :class:`.BaseSampler` when input :class:`.QuantumCircuit` does not
+    have measurements.

test/python/primitives/test_sampler.py

@@ -19,7 +19,6 @@
 from qiskit import QuantumCircuit
 from qiskit.circuit import Parameter
 from qiskit.circuit.library import RealAmplitudes
-from qiskit.exceptions import QiskitError
 from qiskit.extensions.unitary import UnitaryGate
 from qiskit.primitives import Sampler, SamplerResult
 from qiskit.providers import JobStatus, JobV1
@@ -276,6 +275,9 @@ def test_run_errors(self):
         qc2.measure_all()
         qc3 = QuantumCircuit(1)
         qc4 = QuantumCircuit(1, 1)
+        qc5 = QuantumCircuit(1, 1)
+        with qc5.for_loop(range(5)):
+            qc5.h(0)
 
         sampler = Sampler()
         with self.subTest("set parameter values to a non-parameterized circuit"):
@@ -294,10 +296,11 @@ def test_run_errors(self):
             with self.assertRaises(ValueError):
                 _ = sampler.run([qc3], [[]])
         with self.subTest("no measurement"):
-            with self.assertRaises(QiskitError):
-                # The following raises QiskitError because this check is located in
-                # `Sampler._preprocess_circuit`
+            with self.assertRaises(ValueError):
                 _ = sampler.run([qc4], [[]])
+        with self.subTest("no measurement in control flow"):
+            with self.assertRaises(ValueError):
+                _ = sampler.run([qc5], [[]])
 
     def test_run_empty_parameter(self):
         """Test for empty parameter"""