`get_registers` only sees registers added using `add_register`, and not `input_dict`

[lmarti-dev]

Environment

• Qiskit version: 2.2.3
• Python version: 3.11.3
• Operating system: Windows 10

What is happening?

When one creates a Layout for transpilation, using the __init__ argument input_dict, no matter how many registers there are in the Layout, get_registers() returns an empty list. I don't know if this is an intended behaviour, especially since a layout created with input_dict equals a layout whose registers have been added via add_register. If so, it would be useful to specify it in the documentation. One can understand that you're not specifically asking to "add a register" (to self._regs) when using input_dict, but I don't see the use case for creating a layout which uses the registers in ._v2p and ._p2v but keeps its register data empty. The documentation also says that from_list, which is used by the constructor, "populates the layout", but doesn't mention that it keeps the _regs empty.

This causes issues with transpilation passes on third party backends.

How can we reproduce the issue?

from qiskit.transpiler import Layout
from qiskit.circuit import QuantumRegister

q1 = QuantumRegister(3, "q")
q2 = QuantumRegister(4, "ancilla")


input_dict = {j: q for j, q in enumerate([*q1, *q2])}
layout = Layout(input_dict)

regs = layout.get_registers()

layout2 = Layout()
layout2.add_register(q1)
layout2.add_register(q2)

regs2 = layout2.get_registers()

# True
assert layout == layout2

# False
assert regs == regs2

What should happen?

self._regs should contain the QuantumRegisters added by input_dict, or the documentation should warn that there are inequivalent ways to build layouts.

Any suggestions?

No response