`to_qasm()` on classically controlled multi-line gates (e.g. `CCZ`) only guards the first line

[WindOctober]

Describe the issue

When a ClassicallyControlledOperation wraps a sub-operation whose QASM output spans multiple lines—for example, CCZ, which exports as h; ccx; h—to_qasm() prepends a single if (...) prefix to the entire block.

In both OpenQASM 2.0 and 3.0, a brace-less if guards only the single following statement. As a result, every subsequent line executes unconditionally.

This means the exported QASM is semantically different from the original Cirq circuit, with no warning.

Root cause

ClassicallyControlledOperation._qasm_ concatenates the if prefix directly with the raw QASM string returned by the wrapped operation:

# cirq-core/cirq/ops/classically_controlled_operation.py

subop_qasm = protocols.qasm(self._sub_operation, args=args)
...
return f'if ({condition_qasm}) {subop_qasm}'

For CCZ, CCZPowGate._qasm_ returns a multi-line string when exponent == 1:

h ...;
ccx ...;
h ...;

This implementation is in three_qubit_gates.py.

Among the standard-library gates, CCZ is a common multi-line case, while CCX and CSWAP export as single statements.

Since QASM export is an interoperability boundary, downstream tools consuming this output can silently receive a circuit with different semantics.

Explain how to reproduce the bug or problem

import cirq

q = cirq.LineQubit.range(4)

circuit = cirq.Circuit()
circuit.append(cirq.X(q[0]))
circuit.append(cirq.measure(q[3], key='c'))
circuit.append(
    cirq.CCZ(q[0], q[1], q[2]).with_classical_controls('c')
)
circuit.append(cirq.measure(q, key='m'))

print(circuit.to_qasm())

The exported QASM contains:

if (m_c==1) h q[2];
ccx q[0],q[1],q[2];   // executes unconditionally — wrong
h q[2];                // executes unconditionally — wrong

The semantic discrepancy is observable.

Suggested fix

In ClassicallyControlledOperation._qasm_, guard the entire sub-operation rather than only its first statement.

• For OpenQASM 3.0, emit a braced block:

if (m_c==1) {
    h q[2];
    ccx q[0],q[1],q[2];
    h q[2];
}

• For OpenQASM 2.0, which has no equivalent block form for this case, prepend the same if (...) condition to each statement in subop_qasm.

Tell us the version of Cirq where this happens

Reproduced on Cirq v1.7.0 (latest release).

The affected code path is also unchanged on main at commit 8122be0, confirmed by source inspection.

[edubraqd]

Reproduced on main (8e3f40e), both QASM 2.0 and 3.0.

Two notes beyond the report:

1. CCY is affected the same way: CCYPowGate._qasm_ emits sdg; ccx; s, so only the sdg ends up guarded. Among the built-in gates I checked, CCZ and CCY are the multi-statement ones; CCX and CSWAP export as a single statement and are fine.

2. The divergence is observable, not just cosmetic. With the condition never met:

q0, q1, q2, q3 = cirq.LineQubit.range(4)
c = cirq.Circuit(
    cirq.X(q0), cirq.X(q1),
    cirq.measure(q3, key='a'),                       # always 0
    cirq.CCZ(q0, q1, q2).with_classical_controls('a'),
    cirq.measure(q2, key='m'),
)

Simulating c gives m = 0 on every shot. Exporting with c.to_qasm(), re-importing with cirq.contrib.qasm_import.circuit_from_qasm and simulating gives m ∈ {0, 1} — the unguarded ccx flips q2 and the unguarded h leaves it in superposition.

Since the sub-operation's QASM is already one statement per line, guarding each line with the same if (...) prefix in ClassicallyControlledOperation._qasm_ restores the semantics for both 2.0 (no braces available) and 3.0; the existing test_qasm_* tests in classically_controlled_operation_test.py keep passing with that change.