PartialOrd: transitivity and duality are required only if the corresponding impls exist #118108
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cynecx
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…onding impls exist
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| /// - duality: `a < b` if and only if `b > a`. | ||
| /// - **Transitivity**: if `A: PartialOrd<B>` and `B: PartialOrd<C>` and `A: | ||
| /// PartialOrd<C>`, then `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`. | ||
| /// This must also work for longer chains, such as when `A: PartialOrd<B>`, `B: PartialOrd<C>`, |
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| /// This must also work for longer chains, such as when `A: PartialOrd<B>`, `B: PartialOrd<C>`, | |
| /// This should also work for longer chains, such as when `A: PartialOrd<B>`, `B: PartialOrd<C>`, |
(Acknowledging the current state of things here.)
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The current state is that this is a must, no? "The comparison must satisfy, for all [...]".
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(Consolidating this to the similar discussion on PartialEq.)
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I assume we want this to be consistent, despite the different starting points (one PR is relaxing the currently stable requirements/recommendations for I can also merge one PR into the other if you prefer. |
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I've made this part of #115386. |
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Fixes #87067. Currently, not even std itself upholds the requirements documented for
PartialOrd.This is basically doing for
PartialOrdwhat #81198 did forPartialEq. However, #81198 (likely accidentally) significantly weakened the transitivity requirement, which we are avoiding here: as of today, it is the case that ifA: PartialOrd<B>andB: PartialOrd<C>andC: PartialOrd<D>andA: PartialOrd<D>all hold, then ifa < b < c < d, we havea < d. If we just did the same thing as #81198, we would lose that property. Therefore, we explicitly require transitivity for longer chains as well.Libs-api decided here that they are fine with applying #81198 to
PartialOrdas well. I'm still nominating this for them again due to this change in how transitivity is handled.