[Stretch] Support stretch and duration expressions for Delay instruction.
#13853
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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.
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 using classical expressions of type
StretchandDurationas thedurationof aDelayinstruction.This is probably the final PR in the series 🎉. All the missing QPY stuff I'll be doing on the earlier PR branches, and will merge all of that plus fixes forward through to this.
Details and comments
Based on #13852. More readable diff here.
To-do