Comments at the end of an OpenQasm 2.0 circuits gives an error

[francabrera]

Environment

• Qiskit Terra version: 0.25.0
• Python version: 3.10.8
• Operating system:

What is happening?

If there is a comment as the last line of the OpenQasm 2, the from_qasm_str fails with the next error:

Traceback (most recent call last):
  Cell In[16], line 27
    QuantumCircuit.from_qasm_str(qasm_end_comment) # <- fails
  File /opt/conda/lib/python3.10/site-packages/qiskit/circuit/quantumcircuit.py:2537 in from_qasm_str
    return qasm2.loads(
  File /opt/conda/lib/python3.10/site-packages/qiskit/qasm2/__init__.py:452 in loads
    return _parse.from_bytecode(
  File /opt/conda/lib/python3.10/site-packages/qiskit/qasm2/parse.py:214 in from_bytecode
    for op in bc:
QASM2ParseError: '<input>:10,1: needed a start-of-statement token, but instead got /'

This was working in previous versions of Qiskit (can't confirm which version introduced the error).

How can we reproduce the issue?

from qiskit import QuantumCircuit

qasm_end_comment = """
OPENQASM 2.0;
include "qelib1.inc";

qreg q[2];
creg c[2];
h q[0];
cx q[0], q[1];
measure q -> c;
//comment"""

qasm_end_blank_line= """
OPENQASM 2.0;
include "qelib1.inc";

qreg q[2];
creg c[2];
h q[0];
cx q[0], q[1];
measure q -> c;
//comment
"""


QuantumCircuit.from_qasm_str(qasm_end_comment) # <- fails
QuantumCircuit.from_qasm_str(qasm_end_blank_line) # <- works

What should happen?

The OpenQasm is valid, so it should convert it to OpenQasm 2.0.

Any suggestions?

No response

[jakelishman]

This will have been introduced by #9784 and consequently released in Terra 0.24 (so the version before the most recent one). Thanks for the report - looks like the new parser has a slight bug where it doesn't chomp whitespace after skipping a comment correctly, though it's a fun edge case!