🐛 `save_circuit` (QASM3) generates invalid gate: parameter declared but not passed at call site

[J4MMlE]

System and Environment Information

• OS: Linux (NixOS) - kernel 6.19.10-cachyos, x86_64
• Python: 3.13.12
• uv: 0.11.7
• mqt-bench: 2.2.2
• qiskit: 2.4.1

Bug Description

When using save_circuit(..., output_format=OutputFormat.QASM3) for the grover benchmark, the emitted QASM3 file contains a malformed gate definition that causes Qiskit to fail when loading the file.

• Defined (line 1139): gate mcphase_0(_gate_p_0) _gate_q_0, _gate_q_1, ...
• Called without parameter (line 1955): mcphase_0 _gate_q_0, _gate_q_1, ...

Steps to Reproduce

minimal script to reproduce:

from pathlib import Path
from mqt.bench import BenchmarkLevel, get_benchmark
from mqt.bench.output import OutputFormat, save_circuit
from qiskit.qasm3 import load

OUT = Path("grover_10.qasm")

# Step 1: generate the benchmark and export to QASM3
qc = get_benchmark(benchmark="grover", level=BenchmarkLevel.ALG, circuit_size=10)
save_circuit(
    qc, str(OUT.with_suffix("")), BenchmarkLevel.ALG, output_format=OutputFormat.QASM3
)

# Step 2: attempt to load - this fails
try:
    load(OUT)
except Exception as e:
    print(f"Qiskit load fails:\n  {type(e).__name__}: {e}")

[burgholzer]

Thanks for the report! This very much feels like a Qiskit bug to me. Could you check their issue tracker for anything that comes to mind in that regard?
I would have actually hoped that the OpenQASM emitter would just output a ctrl(n-1) @ p(theta) gate for the multi-controlled phase gate.
Maybe one can convince it to do that (somehow)?