qasm() mistranslate the operator QuantumCircuit.ecr

[weucode]

Environment

• Qiskit Terra version: 0.36.1
• Python version: 3.7.4
• Operating system: Windows

What is happening?

The circuit shown below can be translated into OpenQASM successfully,but there exists two rzx gates.The one and only difference between two rzx gates is the parameter of rz gate,one is -pi/4,and another is pi/4.

Here is the error message.

OPENQASM 2.0;
include "qelib1.inc";
gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(-pi/4) q1; cx q0,q1; h q1; }
gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(pi/4) q1; cx q0,q1; h q1; }
gate ecr q0,q1 { rzx(pi/4) q0,q1; x q0; rzx(-pi/4) q0,q1; }
qreg q[2];
ecr q[0],q[1];

Traceback (most recent call last):
  File "20220518.py", line 8, in <module>
    another_circuit.from_qasm_str(qasm_str)
  File "D:\Anaconda3\lib\site-packages\qiskit\circuit\quantumcircuit.py", line 2334, in from_qasm_str
    return _circuit_from_qasm(qasm)
  File "D:\Anaconda3\lib\site-packages\qiskit\circuit\quantumcircuit.py", line 4667, in _circuit_from_qasm
    ast = qasm.parse()
  File "D:\Anaconda3\lib\site-packages\qiskit\qasm\qasm.py", line 53, in parse
    return qasm_p.parse(self._data)
  File "D:\Anaconda3\lib\site-packages\qiskit\qasm\qasmparser.py", line 1137, in parse
    self.parser.parse(data, lexer=self.lexer, debug=self.parse_deb)
  File "D:\Anaconda3\lib\site-packages\ply\yacc.py", line 333, in parse
    return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
  File "D:\Anaconda3\lib\site-packages\ply\yacc.py", line 1120, in parseopt_notrack
    p.callable(pslice)
  File "D:\Anaconda3\lib\site-packages\qiskit\qasm\qasmparser.py", line 612, in p_gate_decl_2
    self.update_symtab(program[0])
  File "D:\Anaconda3\lib\site-packages\qiskit\qasm\qasmparser.py", line 78, in update_symtab
    prev.file,
qiskit.qasm.exceptions.QasmError: "Duplicate declaration for gate 'rzx' at line 4, file .\nPrevious occurrence at line 3, file "

How can we reproduce the issue?

from qiskit import QuantumCircuit

circuit = QuantumCircuit(2)
circuit.ecr(0, 1)
qasm_str = circuit.qasm()
print(qasm_str)
another_circuit = QuantumCircuit(2)
another_circuit.from_qasm_str(qasm_str)
print(another_circuit)

What should happen?

An expected OpenQASM code about QuantumCircuit.ecr is something like this.

gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(param0) q1; cx q0,q1; h q1; }
gate ecr q0,q1 { rzx(pi/4) q0,q1; x q0; rzx(-pi/4) q0,q1; }

Any suggestions?

No response

[jakelishman]

Thanks for the report. Sorry this is a bit bugged out - we have some problems with the OQ 2 exporter when dealing with gates that can take arbitrary parameters but are only used with exact angles, as you've seen.

I unfortunately can't promise that this will be a top priority for the internal team because we're busy with a lot of new features for OpenQASM 3.0 support, but we'd be happy to accept PRs from other people about this issue.

[pollyshaw]

I have a fix for this which is just to include RZX as an atomic gate in qelib1.inc. Is this a suitable approach?

[jakelishman]

For linking purposes, I gave the reason we can't add rzx to qelib1.inc here #8534 (review).