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PartialOrd: transitivity and duality are required only if the corresponding impls exist #118108

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PartialOrd: transitivity and duality are required only if the corresponding impls exist #118108

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16 changes: 10 additions & 6 deletions library/core/src/cmp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -916,14 +916,18 @@ pub macro Ord($item:item) {
/// easy to accidentally make them disagree by deriving some of the traits and manually
/// implementing others.
///
/// The comparison must satisfy, for all `a`, `b` and `c`:
/// The comparison relations must satisfy the following conditions
/// (for all `a`, `b`, `c` of type `A`, `B`, `C`):
///
/// - transitivity: `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
/// - 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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@joshtriplett joshtriplett Dec 5, 2023 •
edited by RalfJung
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(Consolidating this to the similar discussion on PartialEq.)

/// `C: PartialOrd<D>`, and `A: PartialOrd<D>` all exist.
/// - **Duality**: if `A: PartialOrd<B>` and `B: PartialOrd<A>`, then `a < b` if and only if `b > a`.
///
/// Note that these requirements mean that the trait itself must be implemented symmetrically and
/// transitively: if `T: PartialOrd<U>` and `U: PartialOrd<V>` then `U: PartialOrd<T>` and `T:
/// PartialOrd<V>`.
/// Note that the `B: PartialOrd<A>` (dual) and `A: PartialOrd<C>`
/// (transitive) impls are not forced to exist, but these requirements apply
/// whenever they do exist.
///
/// Violating these requirements is a logic error. The behavior resulting from a logic error is not
/// specified, but users of the trait must ensure that such logic errors do *not* result in
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