MeasurementValue raises an error when used as boolean

doc/releases/changelog-dev.md

@@ -261,6 +261,8 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* `MeasurementValue` now raises an error when it is used as a boolean.
+
 * `adjoint_metric_tensor` now works with circuits containing state preparation operations.
   [(#6358)](https://github.com/PennyLaneAI/pennylane/pull/6358)
 

pennylane/measurements/mid_measure.py

@@ -15,8 +15,9 @@
 This module contains the qml.measure measurement.
 """
 import uuid
+from collections.abc import Hashable
 from functools import lru_cache
-from typing import Generic, Hashable, Optional, TypeVar, Union
+from typing import Generic, Optional, TypeVar, Union
 
 import pennylane as qml
 from pennylane.wires import Wires
@@ -456,6 +457,11 @@ def __invert__(self):
         value."""
         return self._apply(qml.math.logical_not)
 
+    def __bool__(self) -> bool:
+        raise ValueError(
+            "The truth value of a MeasurementValue is undefined. To condition on a MeasurementValue, please use qml.cond instead."
+        )
+
     def __eq__(self, other):
         return self._transform_bin_op(lambda a, b: a == b, other)
 

tests/measurements/test_mid_measure.py

@@ -81,6 +81,15 @@ def test_label(self, postselect, reset, expected):
 class TestMeasurementValueManipulation:
     """Test all the dunder methods associated with the MeasurementValue class"""
 
+    def test_error_on_boolean_conversion(self):
+        """Test that an error is raised if a measurement value if used as a boolean."""
+
+        m = MeasurementValue([mp1], lambda v: v)
+
+        with pytest.raises(ValueError, match="The truth value of a MeasurementValue"):
+            if m:
+                return
+
     def test_apply_function_to_measurement(self):
         """Test the general _apply method that can apply an arbitrary function to a measurement."""