QuantumCircuit.draw fails on a condition with a registerless Clbit

[jakelishman]

Information

• Qiskit Terra version: main
• Python version: any
• Operating system: any

What is the current behavior?

The circuit drawers (certainly the text one, prob fail if there is a condition on a Clbit that is not in any registers.

Steps to reproduce the problem

In [1]: from qiskit.circuit import QuantumCircuit, Qubit, Clbit, ClassicalRegister
   ...:
   ...: bits = [Qubit(), Clbit()]
   ...: qc = QuantumCircuit(bits, ClassicalRegister(1))
   ...: qc.x(0).c_if(bits[1], 0)
   ...: qc.draw()
Out[1]: ---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
~/.miniconda3/envs/qiskit/lib/python3.9/site-packages/IPython/core/formatters.py in __call__(self, obj)
    700                 type_pprinters=self.type_printers,
    701                 deferred_pprinters=self.deferred_printers)
--> 702             printer.pretty(obj)
    703             printer.flush()
    704             return stream.getvalue()

~/.miniconda3/envs/qiskit/lib/python3.9/site-packages/IPython/lib/pretty.py in pretty(self, obj)
    392                         if cls is not object \
    393                                 and callable(cls.__dict__.get('__repr__')):
--> 394                             return _repr_pprint(obj, self, cycle)
    395
    396             return _default_pprint(obj, self, cycle)

~/.miniconda3/envs/qiskit/lib/python3.9/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    698     """A pprint that just redirects to the normal repr function."""
    699     # Find newlines and replace them with p.break_()
--> 700     output = repr(obj)
    701     lines = output.splitlines()
    702     with p.group():

~/code/qiskit/terra/qiskit/visualization/text.py in __repr__(self)
    650
    651     def __repr__(self):
--> 652         return self.single_string()
    653
    654     def single_string(self):

~/code/qiskit/terra/qiskit/visualization/text.py in single_string(self)
    658         """
    659         try:
--> 660             return "\n".join(self.lines()).encode(self.encoding).decode(self.encoding)
    661         except (UnicodeEncodeError, UnicodeDecodeError):
    662             warn(

~/code/qiskit/terra/qiskit/visualization/text.py in lines(self, line_length)
    702
    703         try:
--> 704             layers = self.build_layers()
    705         except TextDrawerCregBundle:
    706             self.cregbundle = False

~/code/qiskit/terra/qiskit/visualization/text.py in build_layers(self)
   1132
   1133             for node in node_layer:
-> 1134                 layer, current_connections, connection_label = self._node_to_gate(node, layer)
   1135
   1136                 layer.connections.append((connection_label, current_connections))

~/code/qiskit/terra/qiskit/visualization/text.py in _node_to_gate(self, node, layer)
   1003             # conditional
   1004             op_cond = op.condition
-> 1005             layer.set_cl_multibox(op_cond[0], op_cond[1], top_connect="╨")
   1006             conditional = True
   1007

~/code/qiskit/terra/qiskit/visualization/text.py in set_cl_multibox(self, creg, val, top_connect)
   1343                 bit_index = self._clbit_locations[creg]["index"]
   1344                 label_bool = "= T" if val is True else "= F"
-> 1345                 label = f"{bit_reg.name}_{bit_index} {label_bool}"
   1346                 self.set_clbit(creg, BoxOnClWire(label=label, top_connect=top_connect))
   1347             else:

AttributeError: 'NoneType' object has no attribute 'name'

This happens with output="mpl" and output="latex" as well.

What is the expected behavior?

No error.

Suggested solutions

This is related to a previous issue (#6475), which included a few PRs to the drawers (#6261, #6248, #6259) to add single-bit classical conditioning, but perhaps this was only considering the case of single-qubit conditioning, whereas this is one step further: registerless single-qubit conditioning.

[enavarro51]

@jakelishman I can take a look at this for all 3 drawers. Just doing a quick change on 'mpl' I got this. Not sure what a label should look like for this, but I guess _0 is reasonable.

[jakelishman]

Honestly, I have no opinion on the drawers, and I'm generally prepared to go with whatever label you suggest seems sensible. I guess as long as it in some way matches the wire's label on the very left edge I'm happy. Thanks for taking it!