Standard Gates in 'qelib1.inc' Not Recognized by qiskit.qasm2.loads

[p51lee]

Environment

• Qiskit version: 1.0.2
• Python version: 3.10.9
• Operating system: MacOS 14.4

What is happening?

qiskit.qasm2.loads is not recognizing standard gates provided in qelib1.inc.

How can we reproduce the issue?

To reproduce the issue, execute the following Python code:

import qiskit

qasm = """
OPENQASM 2.0;
include "qelib1.inc";
qreg q[2];
swap q[0], q[1];
"""
circuit = qiskit.qasm2.loads(qasm)

Running this code results in a QASMParseError:

qiskit.qasm2.exceptions.QASM2ParseError: "<input>:10,6: 'swap' is not defined in this scope"

This error is also happening with other standard gates from qelib1.inc:

• u0
• u
• p
• sx
• sxdg
• swap
• cswap
• crx
• cry
• cp
• csx
• cu
• rxx
• rzz
• rccx
• rc3x
• c3x
• c3sqrtx
• c4x

What should happen?

The parser should recognize and correctly process these gates as pre-defined, analogous to how it handles the Hadamard gate.

Any suggestions?

No response

[jakelishman]

The version of the "qelib1.inc" file you've linked here is actually a mutant version that Qiskit amended over time, and does not reflect the original definition of the file - all the gates you've commented on here are legacy Qiskit extensions to it. The qasm2.load and qasm2.loads parsers both treat qelib1.inc exactly as specified in the original paper, and that documentation also explains the differences between the "real" and "legacy" versions.

Unfortunately, qiskit.qasm2.dumps still outputs assuming the mutant version of qelib1.inc right now - we have a discussion (#10737) on fixing that situation, but we keep struggling to find time to prioritise it. In the mean time, you can make qasm2.loads (and load) understand the whole legacy file by doing:

from qiskit import qasm2

qasm2.loads(program, custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS)

[p51lee]

Thank you for �the clarification. Indeed, I encountered issues when trying to dump and load QASM programs. I'll use LEGACY_CUSTOM_INSTRUCTIONS in the meantime.