Sampling issue of Qasm simulator at BasicAer

[chunfuchen]

Information

• Qiskit Terra version: the master branch
• Python version: 3.7
• Operating system: macOS 10.14

What is the current behavior?

Produce the correct measurement result

Steps to reproduce the problem

from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit import BasicAer, Aer, execute
basicaer = BasicAer.get_backend('qasm_simulator')
aer = Aer.get_backend('qasm_simulator')
qr = QuantumRegister(2)
cr = ClassicalRegister(1)
qc = QuantumCircuit(qr, cr)
qc.h(qr[0])
qc.x(qr[1])
qc.measure(qr[1], cr[0])

basicaer_result = execute(qc, backend=basicaer).result().get_counts()
aer_result = execute(qc, backend=aer).result().get_counts()

print(basicaer_result)
print(aer_result)

What is the expected behavior?

the results should be {'1': 1024} but BasicAer returns wrong results.

Suggested solutions

No idea but it is related to the PR #2735 since before this PR, basic aer works correctly.
Furthermore, if all quantum registers are measured, the basic aer works okay. It seems that this issue only happened when only a part of quantum registers are measured.