Fix the default value of qubits_len for the qinst primitive (#496)

Two issues are being fixed:
- The default value of `-1`, intended to represent all arguments are
  qubits, does not actually work. Instead, it introduces a off by one
  error, cutting off one of the qubit values.
- The abstract evaluation did not work with the default value, and
  instead assumed the value was always set manually.

Credit to @positr0nium for unearthing this :)

---------

Co-authored-by: Ali Asadi <10773383+maliasadi@users.noreply.github.com>

doc/changelog.md

@@ -162,13 +162,18 @@
   implemented in Catalyst's runtime:
   * `int64_t __catalyst__rt__array_get_size_1d(QirArray *)`
   * `int8_t *__catalyst__rt__array_get_element_ptr_1d(QirArray *, int64_t)`
-  
+
   and the following functions were removed since the frontend does not generate them
   * `QirString *__catalyst__rt__qubit_to_string(QUBIT *)`
   * `QirString *__catalyst__rt__result_to_string(RESULT *)`
 
 <h3>Bug fixes</h3>
 
+* Fix an issue when no qubit number was specified for the `qinst` primitive. The primitive now
+  correctly deduces the number of qubits when no gate parameters are present. This change is not
+  user facing.
+  [(#496)](https://github.com/PennyLaneAI/catalyst/pull/496)
+
 * Fix the scatter operation lowering when `updatedWindowsDim` is empty.
   [(#475)](https://github.com/PennyLaneAI/catalyst/pull/475)
 
@@ -249,9 +254,9 @@ Haochen Paul Wang.
   def f(x):
       def cnot_loop(j):
           qml.CNOT(wires=[j, jnp.mod((j + 1), 4)])
-  
+
       for_loop(0, 4, 1)(cnot_loop)()
-  
+
       return qml.expval(qml.PauliZ(0))
   ```
 

frontend/catalyst/jax_primitives.py

@@ -682,11 +682,11 @@ def _qinsert_lowering(
 # qinst
 #
 @qinst_p.def_abstract_eval
-def _qinst_abstract_eval(*qubits_or_params, op=None, qubits_len=-1):
-    for idx in range(qubits_len):
-        qubit = qubits_or_params[idx]
+def _qinst_abstract_eval(*qubits_or_params, op=None, qubits_len=None):
+    qubits = qubits_or_params[:qubits_len]
+    for qubit in qubits:
         assert isinstance(qubit, AbstractQbit)
-    return (AbstractQbit(),) * qubits_len
+    return (AbstractQbit(),) * len(qubits)
 
 
 @qinst_p.def_impl
@@ -695,7 +695,7 @@ def _qinst_def_impl(ctx, *qubits_or_params, op, qubits_len):  # pragma: no cover
 
 
 def _qinst_lowering(
-    jax_ctx: mlir.LoweringRuleContext, *qubits_or_params: tuple, op=None, qubits_len=-1
+    jax_ctx: mlir.LoweringRuleContext, *qubits_or_params: tuple, op=None, qubits_len=None
 ):
     ctx = jax_ctx.module_context.context
     ctx.allow_unregistered_dialects = True

frontend/test/pytest/test_jax_primitives.py

@@ -0,0 +1,36 @@
+# Copyright 2024 Xanadu Quantum Technologies Inc.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for the JAX primitives module."""
+
+import pytest
+
+from catalyst.jax_primitives import AbstractQbit, qinst_p
+
+
+class TestQinstPrim:
+    """Test the quantum instruction primitive."""
+
+    def test_abstract_eval_no_len(self):
+        """Test that the number of qubits is properly deduced when not set automatically."""
+
+        qb0, qb1 = (AbstractQbit(),) * 2
+        result = qinst_p.abstract_eval(qb0, qb1, op="GarbageOp")[0]
+
+        assert len(result) == 2
+        assert all(isinstance(r, AbstractQbit) for r in result)
+
+
+if __name__ == "__main__":
+    pytest.main(["-x", __file__])