BlockCollapser fails for ClassicalRegister conditions

[chriseclectic]

Environment

• Qiskit Terra version: 0.24 and 0.25rc1
• Python version: All
• Operating system: All

What is happening?

The BlockCollapser class fails for blocks containing multiple nodes with full ClassicalRegister conditions on the same registers.

How can we reproduce the issue?

Minimal example

from qiskit import QuantumCircuit
from qiskit.converters import circuit_to_dag
from qiskit.dagcircuit.collect_blocks import BlockCollector, BlockCollapser

qc = QuantumCircuit(1, 2)
qc.x(0).c_if(qc.cregs[0], 0)
qc.y(0).c_if(qc.cregs[0], 1)

dag = circuit_to_dag(qc)
blocks = BlockCollector(dag).collect_all_matching_blocks(
    lambda _: True, split_blocks=False, min_block_size=1
)
dag = BlockCollapser(dag).collapse_to_operation(blocks, lambda circ : circ.to_instruction())

---------------------------------------------------------------------------
CircuitError                              Traceback (most recent call last)
Cell In [5], line 14
      8 dag = circuit_to_dag(qc)
     10 blocks = BlockCollector(dag).collect_all_matching_blocks(
     11     lambda _: True, split_blocks=False, min_block_size=1
     12 )
---> 14 dag = BlockCollapser(dag).collapse_to_operation(blocks, lambda circ : circ.to_instruction())

File ~/mambaforge/envs/pec/lib/python3.9/site-packages/qiskit/dagcircuit/collect_blocks.py:368, in BlockCollapser.collapse_to_operation(self, blocks, collapse_fn)
    366 # Add classical registers used in conditions over registers
    367 for reg in cur_clregs:
--> 368     qc.add_register(reg)
    370 # Construct a quantum circuit from the nodes in the block, remapping the qubits.
    371 wire_pos_map = {qb: ix for ix, qb in enumerate(sorted_qubits)}

File ~/mambaforge/envs/pec/lib/python3.9/site-packages/qiskit/circuit/quantumcircuit.py:1425, in QuantumCircuit.add_register(self, *regs)
   1421 for register in regs:
   1422     if isinstance(register, Register) and any(
   1423         register.name == reg.name for reg in self.qregs + self.cregs
   1424     ):
-> 1425         raise CircuitError('register name "%s" already exists' % register.name)
   1427     if isinstance(register, AncillaRegister):
   1428         for bit in register:

CircuitError: 'register name "c" already exists'

What should happen?

No error.

Any suggestions?

The collapse_to_operation method is incorrectly collecting the full classical register from all conditional nodes in a block as a list, when it should be a set to avoid duplication.

[jakelishman]

Thanks for the report, and yeah, that does appear to be the direct reason.

[cjh1]

I would like to take a crack at this one, if someone can assign it to me.