[DNM][SR-14665] add default == implementation for Comparable
#39371
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Currently `Comparable` inherits from `Equatable`, but does not provide a default implementation for `==`, so the compiler synthesizes one composed of [member-wise `==`s](https://github.com/apple/swift-evolution/blob/main/proposals/0185-synthesize-equatable-hashable.md#implementation-details). 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: ```swift 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. ``` Relevant forums discussion: https://forums.swift.org/t/add-default-implementation-of-to-comparable/48832 This bug has previously resulted in incorrect semantic version comparison in SwiftPM (swiftlang/swift-package-manager#3486 and swiftlang/swift-tools-support-core#214)
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@swift-ci Please Test Source Compatibility |
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Do not merge.
An alternative to #39047.
As discussed in the forums thread, this might be source-breaking, so this PR is just doing a compatibility suite test.