[Stretch] Add stretch variable support to QuantumCircuit.
#13852
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This reverts commit e2b3221.
Types that have some natural order no longer have an ordering when one of them is strictly greater but has an incompatible const-ness (i.e. when the greater type is const but the other type is not).
We need to reject types with const=True in QPY until it supports them. For now, I've also made the Index and shift operator constructors lift their RHS to the same const-ness as the target to make it less likely that existing users of expr run into issues when serializing to older QPY versions.
This is probably a better default in general, since we don't really have much use for const except for timing stuff.
Since we're going for using a Cast node when const-ness differs, this will be fine.
I wasn't going to have this, but since we have DANGEROUS Float => Int, and we have Int => Bool, I think this makes the most sense.
A Stretch can always represent a Duration (it's just an expression without any unresolved stretch variables, in this case), so we allow implicit conversion from Duration => Stretch. The reason for a separate Duration type is to support things like Duration / Duration => Float. This is not valid for stretches in OpenQASM (to my knowledge).
Also adds support to expr.lift to create a value expression of type types.Duration from an instance of qiskit.circuit.Duration.
I can't really test Stretch yet since we can't add them to circuits until a later PR.
The best I can do to test these right now (before Delay is made to accept timing expressions) is to use them in comparison operations (will be in a follow up commit).
We decided to punt on the idea of assigning const-typed variables, so we don't support declaring stretch expressions via add_var or the 'declarations' argument to QuantumCircuit. We also don't allow const 'inputs'.
At the moment, we track stretches as variables, but these will eventually make their way here during circuit copy etc., so we need to support them even though they're const.
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Summary
Adds support for adding new uninitialized
stretchvariables to a circuit viaQuantumCircuit::add_stretch. The returned variable follows the same scoping rules as other classical circuit variables.Stretch variables can be used to initialize other stretch variables as well in an expression via the existingEDIT: we decided to punt on this, since it'd likely require inline initializers when writing to QASM, which isn't something we've done before, so the functionality has been removed.QuantumCircuit::add_varmethod.Details and comments
Based on #13850. See the readable diff here.
QuantumCircuit.add_uninitialized_varto be allowed even thoughtypes.Stretchis always const (...which is normally blocked to prevent users from creating uninitialized constants).QuantumCircuit"input" variables, since we don't allow const input variables (this can be relaxed in the future, if needed).DelayandBoxstart using them in their duration expressions.To-do