Improve error message for invalid measurement in adjoint() region (#1425

) **Context:** Improve the error message when using measure within an `adjoin()` region. **Example** ```python import pennylane as qml from pennylane import qjit from catalyst import measure dev = qml.device("lightning.qubit", wires=1) @qjit @qml.qnode(dev) def circuit(): def foo(): measure(wires=0) qml.adjoint(foo)() return qml.expval(qml.PauliZ(0)) print(circuit()) ``` **Description of the Change:** Write and insert an error message `"Measurements are not invertible and cannot be used within an adjoint() region."` **Related GitHub Issues:** #1188 --------- Co-authored-by: David Ittah <dime10@users.noreply.github.com>
PennyLaneAI · Jan 13, 2025 · 55d424b · 55d424b
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diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -20,9 +20,13 @@
 * `from_plxpr` now uses the `qml.capture.PlxprInterpreter` class for reduced code duplication.
   [(#1398)](https://github.com/PennyLaneAI/catalyst/pull/1398)
 
+* Improve the error message for invalid measurement in `adjoin()` or `ctrl()` region.
+  [(#1425)](https://github.com/PennyLaneAI/catalyst/pull/1425)
+
 * Replace `ValueRange` with `ResultRange` and `Value` with `OpResult` to better align with the semantics of `**QubitResult()` functions like `getNonCtrlQubitResults()`. This change ensures clearer intent and usage. Improve the `matchAndRewrite` function by using `replaceAllUsesWith` instead of for loop.
   [(#1426)](https://github.com/PennyLaneAI/catalyst/pull/1426)
 
+
 <h3>Documentation 📝</h3>
 
 <h3>Contributors ✍️</h3>
@@ -31,4 +35,4 @@ This release contains contributions from (in alphabetical order):
 
 Christina Lee
 Mehrdad Malekmohammadi
-Sengthai Heng
+Sengthai Heng
diff --git a/frontend/catalyst/api_extensions/quantum_operators.py b/frontend/catalyst/api_extensions/quantum_operators.py
@@ -712,14 +712,15 @@ def ctrl_distribute(
 def _check_no_measurements(tape: QuantumTape) -> None:
     """Check the nested quantum tape for the absense of quantum measurements of any kind"""
 
-    msg = "Quantum measurements are not allowed"
-
     if len(tape.measurements) > 0:
-        raise ValueError(msg)
+        raise ValueError("Measurement process cannot be used within an adjoint() or ctrl() region.")
     for op in tape.operations:
         if has_nested_tapes(op):
             for r in [r for r in op.regions if r.quantum_tape is not None]:
                 _check_no_measurements(r.quantum_tape)
         else:
             if isinstance(op, MidCircuitMeasure):
-                raise ValueError(msg)
+                raise ValueError(
+                    "Mid-circuit measurements cannot be used "
+                    "within an adjoint() or ctrl() region."
+                )
diff --git a/frontend/test/pytest/test_adjoint.py b/frontend/test/pytest/test_adjoint.py
@@ -245,7 +245,7 @@ def func():
             qml.RX(np.pi / 2, wires=0)
             qml.sample()
 
-        with pytest.raises(ValueError, match="Quantum measurements are not allowed"):
+        with pytest.raises(ValueError, match="Measurement process cannot be used"):
 
             @qjit
             @qml.qnode(qml.device("lightning.qubit", wires=2))

diff --git a/frontend/test/pytest/test_quantum_control.py b/frontend/test/pytest/test_quantum_control.py
@@ -290,7 +290,7 @@ def _func1():
             C_ctrl(_func1, control=[1], control_values=[True])()
             return qml.state()
 
-        with pytest.raises(ValueError, match="measurements are not allowed"):
+        with pytest.raises(ValueError, match="Mid-circuit measurements cannot be used"):
             qjit(circuit)(0.1)
 
     def test_qctrl_no_end_circuit_measurements(self, backend):
@@ -305,7 +305,7 @@ def _func1():
             C_ctrl(_func1, control=[1], control_values=[True])()
             return qml.state()
 
-        with pytest.raises(ValueError, match="measurements are not allowed"):
+        with pytest.raises(ValueError, match="Measurement process cannot be used"):
             qjit(circuit)(0.1)
 
     def test_qctrl_wires(self, backend):