Information

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

What is the current behavior?

If two classical registers have a non-empty intersection of clbits, it's possible to have InstructionSet.c_if resolve an integer index to a different value than the original QuantumCircuit would.

Steps to reproduce the problem

In [1]: from qiskit.circuit import QuantumCircuit, ClassicalRegister, Clbit, Qubit
   ...: clbits = [Clbit() for _ in [None]*3]
   ...: # Two overlapping classical registers.
   ...: cr1 = ClassicalRegister(bits=clbits[:2])
   ...: cr2 = ClassicalRegister(bits=clbits[1:])
   ...:
   ...: qc = QuantumCircuit([Qubit()], cr1, cr2)
   ...: qc.x(0).c_if(2, 0)
   ...: qc.measure(0, 2)
   ...:
   ...: clbit_from_c_if = qc.data[0][0].condition[0]
   ...: clbit_from_measure = qc.data[1][2][0]
   ...: clbit_from_c_if is clbit_from_measure
Out[1]: False

What is the expected behavior?

They should resolve to the same clbit, because they use the same index.

Suggested solutions

Instead of storing references to the classical registers in the instruction set, store a reference to the circuit, or a bound callable that came from the circuit to resolve the indices.