[Capture] Add error if qml.cond predicate is not a scalar

doc/releases/changelog-dev.md

@@ -121,6 +121,9 @@
   technically a callable that returns an `X` operator.
   [(#8060)](https://github.com/PennyLaneAI/pennylane/pull/8060)
 
+* With program capture, an error is now raised if the conditional predicate is not a scalar.
+  [(#8066)](https://github.com/PennyLaneAI/pennylane/pull/8066)
+
 <h4>OpenQASM-PennyLane interoperability</h4>
 
 * The :func:`qml.from_qasm3` function can now convert OpenQASM 3.0 circuits that contain

pennylane/ops/op_math/condition.py

@@ -282,6 +282,8 @@ def __call_capture_enabled(self, *args, **kwargs):
         abstracted_axes, abstract_shapes = qml.capture.determine_abstracted_axes(args)
 
         for pred, fn in branches:
+            if (pred_shape := qml.math.shape(pred)) != ():
+                raise ValueError(f"Condition predicate must be a scalar. Got {pred_shape}.")
             conditions.append(pred)
             if fn is None:
                 jaxpr_branches.append(None)

tests/capture/test_capture_cond.py

@@ -58,6 +58,16 @@ def false_fn(arg):
     return true_fn, false_fn, elif_fn1, elif_fn2, elif_fn3, elif_fn4
 
 
+def test_bad_predicate_shape():
+    """Test that an error is raised if the predicate is not a scalar."""
+
+    def f():
+        qml.cond(np.array([0, 0]), qml.X, qml.Z)(0)
+
+    with pytest.raises(ValueError, match="predicate must be a scalar"):
+        jax.make_jaxpr(f)()
+
+
 @pytest.mark.parametrize("decorator", [True, False])
 class TestCond:
     """Tests for conditional functions using qml.cond."""