Qiskit / qiskit / 7047396072

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Committed by web-flow

Commit Message

Ensure `QuantumCircuit.append` validates captures in control-flow (#10974)

* Add definition of `Store` instruction

This does not yet add the implementation of `QuantumCircuit.store`,
which will come later as part of expanding the full API of
`QuantumCircuit` to be able to support these runtime variables.

The `is_lvalue` helper is added generally to the `classical.expr` module
because it's generally useful, while `types.cast_kind` is moved from
being a private method in `expr` to a public-API function so `Store` can
use it.  These now come with associated unit tests.

* Add variable-handling methods to `QuantumCircuit`

This adds all the new `QuantumCircuit` methods discussed in the
variable-declaration RFC[^1], and threads the support for them through
the methods that are called in turn, such as `QuantumCircuit.append`.
It does yet not add support to methods such as `copy` or `compose`,
which will be done in a follow-up.

The APIs discussed in the RFC necessitated making `Var` nodes hashable.
This is done in this commit, as it is logically connected.  These nodes
now have enforced immutability, which is technically a minor breaking
change, but in practice required for the properties of such expressions
to be tracked correctly through circuits.

A helper attribute `Var.standalone` is added to unify the handling of
whether a variable is an old-style existing-memory wrapper, or a new
"proper" variable with its own memory.

[^1]: https://github.com/Qiskit/RFCs/pull/50

* Support manual variables `QuantumCircuit` copy methods

This commit adds support to the `QuantumCircuit` methods `copy` and
`copy_empty_like` for manual variables.  This involves the non-trivial
extension to the original RFC[^1] that variables can now be
uninitialised; this is somewhat required for the logic of how the
`Store` instruction works and the existence of
`QuantumCircuit.copy_empty_like`; a variable could be initialised with
the result of a `measure` that no longer exists, th... (continued)

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