[SR-14665] Synthesised Equatable may be incorrect for manually implemented Comparable

[WowbaggersLiquidLunch]

Previous ID	SR-14665
Radar	None
Original Reporter	@WowbaggersLiquidLunch
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DerivedConformance
Assignee	None
Priority	Medium

md5: 7c4663ac0f4ead1b689e43c0ea8393a3

relates to:

• SR-11588 Warn about derived Hashable implementation if there's a custom Equatable

Issue Description:

Currently Comparable inherits from Equatable, but does not provide a default implementation for ==, so the compiler synthesizes one composed of member-wise ==s. This leads to a problem where if a type's < is not composed of member-wise inequalities, then <, >, and == can all evaluate to false for some pairs of values, contradicting Comparable's documentation:

Types with Comparable conformance implement the less-than operator (<) and the equal-to operator (==). These two operations impose a strict total order on the values of a type, in which exactly one of the following must be true for any two values a and b:

• a == b

• a < b

• b < a

For example:

struct Length: Comparable {
    enum Unit: Double, Comparable {
        case mm = 0.001
        case m = 1
        case banana = 0.178
    }
    
    let magnitude: Double
    let unit: Unit
    
    static func < (lhs: Self, rhs: Self) -> Bool {
        lhs.magnitude * lhs.unit.rawValue < rhs.magnitude * rhs.unit.rawValue
    }
}

let aBanana = Length(magnitude: 1, unit: .banana)
let oneBanana = Length(magnitude: 0.178, unit: .m)

print(aBanana < oneBanana)  // prints "false", because Length's < says so.
print(aBanana > oneBanana)  // prints "false", because Comparable's default implementation of >(a,b) is <(b,a).
print(aBanana == oneBanana) // prints "false", because the 2 Length instances are not member-wise equal.

Forums discussion: https://forums.swift.org/t/add-default-implementation-of-to-comparable/48832

[WowbaggersLiquidLunch]

submitted a draft PR #39047 #39047

[WowbaggersLiquidLunch]

I haven't had time in the past few months to continue working on this, and don't think I will be anytime soon, so I unassigned myself. If anyone wants to take over, please feel free to do so.