Fix pickling of conditioned custom gates from OpenQASM 2 import

[jakelishman]

Summary

The condition of a gate defined by a gate statement in an OpenQASM 2 program was not previously exported as part of its state during pickling, causing pickle roundtrips or deepcopies to lose the condition on the new copy.

Details and comments

Fix #11174.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 6733858362

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 11 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.007%) to 86.889%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	93.76%
crates/qasm2/src/lex.rs	4	91.92%
crates/qasm2/src/parse.rs	6	97.6%

Totals
Change from base Build 6731270262:	0.007%
Covered Lines:	74033
Relevant Lines:	85204

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💛 - Coveralls

[ElePT]

I am not an expert on OpenQASM but the fix looks reasonably simple and the test covers all the potential pain points described in #11174 (deep-copying and pickling), so LGTM. Out of curiosity, are there ways to define conditional custom gates without a gate statement? asking because of the way the reno is phrased.

[jakelishman]

Out of curiosity, are there ways to define conditional custom gates without a gate statement?

No, the OQ2 language doesn't support that - the phrasing in the release note is supposed to imply

Previously, any conditional custom gate (i.e. those defined by a gate statement in an OpenQASM 2 file) would lose its condition when copied or pickled.

and not

Previously, any conditional custom gate (e.g. those defined by a gate statement in an OpenQASM 2 file) would lose its condition when copied or pickled.

I can change it if you prefer to be clearer.

[ElePT]

I think that is ok, it's probably clear to the target audience of the reno and I was just lacking background knowledge.