Fix handling of cargs in BasicQisVisitor

The unit tests in this commit fail without the change made to file
src/qiskit_qir/visitor.py.

src/qiskit_qir/visitor.py

@@ -199,7 +199,7 @@ def process_composite_instruction(
             )
         for (inst, i_qargs, i_cargs) in subcircuit.data:
             mapped_qbits = [qargs[subcircuit.qubits.index(i)] for i in i_qargs]
-            mapped_clbits = [cargs[subcircuit.clbits.index] for i in i_cargs]
+            mapped_clbits = [cargs[subcircuit.clbits.index(i)] for i in i_cargs]
             _log.debug(
                 f"Processing sub-instruction {inst.name} with mapped qubits {mapped_qbits}"
             )

tests/test_circuits/__init__.py

@@ -12,6 +12,7 @@
     "teleport",
     "unroll",
     "teleport_with_subroutine",
+    "measure_x_as_subroutine"
 ] + random_fixtures
 
 noop_tests = ["bernstein_vazirani_with_delay", "ghz_with_delay"]

tests/test_circuits/basic_circuits.py

@@ -117,3 +117,18 @@ def ghz_with_delay():
     circuit.measure([0, 1, 2], [0, 1, 2])
 
     return circuit
+
+@pytest.fixture()
+def measure_x_as_subroutine():
+    measure_x_circuit = QuantumCircuit(1, 1, name='measure_x')
+    measure_x_circuit.h(0)
+    measure_x_circuit.measure(0, 0)
+    measure_x_circuit.h(0)
+    measure_x_gate = measure_x_circuit.to_instruction()
+    qq = QuantumRegister(1, name="qq")
+    cr = ClassicalRegister(1, name="cr")
+    circuit = QuantumCircuit(qq, cr)
+    circuit.name = "Qiskit Sample - Measure in the X-basis as a subroutine"
+    circuit.append(measure_x_gate, [0], [0])
+
+    return circuit