Swift Equatable on a protocol

Question

I don\'t think this can be done but I\'ll ask anyway. I have a protocol:

protocol X {}

And a class:

class Y:X {}

Answer 1

Determining equality across conformances to a Swift protocol is possible without type erasure if:

• you are willing to forgo the operator syntax (i.e. call isEqual(to:) instead of ==)
• you control the protocol (so you can add an isEqual(to:) func to it)

import XCTest

protocol Shape {
    func isEqual (to: Shape) -> Bool
}

extension Shape where Self : Equatable {
    func isEqual (to: Shape) -> Bool {
        return (to as? Self).flatMap({ $0 == self }) ?? false
    }
}

struct Circle : Shape, Equatable {
    let radius: Double
}

struct Square : Shape, Equatable {
    let edge: Double
}

class ProtocolConformanceEquality: XCTestCase {

    func test() {
        // Does the right thing for same type
        XCTAssertTrue(Circle(radius: 1).isEqual(to: Circle(radius: 1)))
        XCTAssertFalse(Circle(radius: 1).isEqual(to: Circle(radius: 2)))

        // Does the right thing for different types
        XCTAssertFalse(Square(edge: 1).isEqual(to: Circle(radius: 1)))
    }

}

Any conformances don't conform to Equatable will need to implement isEqual(to:) themselves