Qiskit / qiskit / 24563004926

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Committed by web-flow

Commit Message

Fix panic in `RemoveIdentityEquivalent` with `ParameterVector` global phase (#16054) (#16055)

If a circuit has a global phase containing a `ParameterVectorElement`,
the resulting `SymbolExpr` will transitively contain a `Py<PyAny>`
pointing back to the `ParameterVector` object.  When adding to the
global phase, a name/symbol-map clone occurs, which requires us to be
attached to the Python interpreter.  Since `RemoveIdentityEquivalent`
manually detaches from the Python interpreter, we may attempt to
increment the refcount without being attached.

This patch is intended to be minimal for backport to 2.4; there are
several more invasive changes we could make (and largely _should_ make)
to make `ParameterExpression` safer and more performant, and in general
it is becoming clear we still have a lot of work to do to be able to
remove the `py-clone` feature use of PyO3 and let the compiler catch
these mistakes for us.

(cherry picked from commit 16c808857)

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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