[Stretch] Add +, -, *, / operators for classical expressions.
#13850
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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).
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Summary
Adds new arithmetic operators for use with numeric and a timing expressions. These are needed to express relationships between stretch variables and durations.
Details and comments
Based on #13844. More readable diff here..
UintandFloat, but these cannot be intermixed. You must first cast one to the other explicitly, sinceFloat => UintandUint => Floatare both considered dangerous cast kinds. If twoUints of different width are used, the wider one is chosen for the expression's output type.DurationandStretch, and these can be intermixed. If either operand is aStretch, both operands are coerced toStretch.Floatoperands. Currently, a constUintmust first be cast to a constFloatto be used as a timing multiplier.DurationorStretch, since they are inherently const.Floatoperands, but only where the timing operand is in theleftposition. It is also supported between twoDurationtypes, in which case the result is a constFloat.Stretchtypes cannot be divided.To-do