Qiskit's circuit_to_qasm_string functionality can't handle qiskit.circuit.library.GR circuit

[vtomole]

Environment

• Qiskit Terra version: 0.18.3
• Python version: Python 3.8.10
• Operating system: Ubuntu 20.04.4 LTS

What is happening?

Converting qiskit.circuit.library.GR capitalizes the GR instruction (GR) when it should be gr. GR is not valid QASM. It also defines the GR gate in int parameters instead of ID.

How can we reproduce the issue?

import qiskit
circuit = qiskit.circuit.library.GR(2, 0.1, 0.2)  # (note GR() outputs a QuantumCircuit, not a Gate)
qasm_str = circuit.qasm()
print(qasm_str)  
# prints:
# """
# OPENQASM 2.0;
# include "qelib1.inc";
# gate r(param0,param1) q0 { u3(0.1,-1.3707963,1.3707963) q0; }
# gate GR(0.10, 0.20) q0,q1 { r(0.1,0.2) q0; r(0.1,0.2) q1; }
# qreg q[2];
# GR(0.10, 0.20) q[0],q[1];
# """

_ = qiskit.QuantumCircuit.from_qasm_str(qasm_str)  # raises: "qiskit.qasm.exceptions.QasmError: 'Unable to match any token rule, got -->G<-- Check your OPENQASM source and any include statements.'"

Converting the qasm definition to gr comes up with a different error

qasm_str =  """
    OPENQASM 2.0;
    include "qelib1.inc";
    gate r(param0,param1) q0 { u3(0.1,-1.3707963,1.3707963) q0; }
    gate gr(0.10, 0.20) q0,q1 { r(0.1,0.2) q0; r(0.1,0.2) q1; }
    """

_ = qiskit.QuantumCircuit.from_qasm_str(qasm_str) # raises qiskit.qasm.exceptions.QasmError: "Expected an ID, received '0.10'"

What should happen?

The above snippets shouldn't throw exceptions.

Any suggestions?

No response

[jakelishman]

Thanks for the report! Please could you try this on a more recent version of Terra? I believe this should be fixed by #7157, which was released in Terra 0.19. I think the bug came because the GR gate used to say its "name" was GR(0.1, 0.2) and that it had zero parameters - the QASM 2 exporter used to just take the name verbatim (which was clearly incorrect), so here it ends up outputting something that looks like a gate definition, but is completely wrong.

At any rate, with the current main branch of Terra I get:

from qiskit.circuit.library import GR
print(GR(2, 0.1, 0.2).qasm())

OPENQASM 2.0;
include "qelib1.inc";
gate r(param0,param1) q0 { u3(0.1,-1.3707963,1.3707963) q0; }
gate gate_GR_0_10__0_20_ q0,q1 { r(0.1,0.2) q0; r(0.1,0.2) q1; }
qreg q[2];
gate_GR_0_10__0_20_ q[0],q[1];

[vtomole]

Yup, this is fixed by terra version 0.19. Thanks!