From 7ee33332fc86205c5b667d0ca7e2bd29dd725c1f Mon Sep 17 00:00:00 2001
From: Jake Lishman <jake.lishman@ibm.com>
Date: Fri, 5 Jan 2024 13:26:55 +0000
Subject: [PATCH] Ensure error-checking happens before any modification

After the move of `QuantumCircuit.data` to Rust-space, some of the state
modification had started happening before the error checking, which
could potentially leave an in-place modification in a partially applied
state if an exception triggered during processing.

This also ensures that the qubits and clbits arguments are checked, even
if the actual data being composed is empty.
---
 qiskit/circuit/quantumcircuit.py | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/qiskit/circuit/quantumcircuit.py b/qiskit/circuit/quantumcircuit.py
index 2800f539b650..1d03ea090902 100644
--- a/qiskit/circuit/quantumcircuit.py
+++ b/qiskit/circuit/quantumcircuit.py
@@ -997,21 +997,6 @@ def compose(
                 "Trying to compose with another QuantumCircuit which has more 'in' edges."
             )
 
-        for gate, cals in other.calibrations.items():
-            dest._calibrations[gate].update(cals)
-
-        dest.global_phase += other.global_phase
-
-        if not other.data:
-            # Nothing left to do. Plus, accessing 'data' here is necessary
-            # to trigger any lazy building since we now access '_data'
-            # directly.
-            return None if inplace else dest
-
-        # The 'qubits' and 'clbits' used for 'dest'.
-        mapped_qubits: list[Qubit]
-        mapped_clbits: list[Clbit]
-
         # Maps bits in 'other' to bits in 'dest'. Used only for
         # adjusting bits in variables (e.g. condition and target).
         edge_map: dict[Qubit | Clbit, Qubit | Clbit] = {}
@@ -1047,6 +1032,21 @@ def compose(
                 )
             edge_map.update(zip(other.clbits, dest.cbit_argument_conversion(clbits)))
 
+        for gate, cals in other.calibrations.items():
+            dest._calibrations[gate].update(cals)
+
+        dest.global_phase += other.global_phase
+
+        if not other.data:
+            # Nothing left to do. Plus, accessing 'data' here is necessary
+            # to trigger any lazy building since we now access '_data'
+            # directly.
+            return None if inplace else dest
+
+        # The 'qubits' and 'clbits' used for 'dest'.
+        mapped_qubits: list[Qubit]
+        mapped_clbits: list[Clbit]
+
         variable_mapper = _classical_resource_map.VariableMapper(
             dest.cregs, edge_map, dest.add_register
         )