swift: why need to conform equatable when overload the "==" operator?

Question

I'm learning swift and read a topic about operator overloading in extensions, which likes:

extension StreetAddress: Equatable {
    static func == (lhs: StreetAddress, rhs: StreetAddress) -> Bool {
        return
            lhs.number == rhs.number &&
            lhs.street == rhs.street &&
            lhs.unit == rhs.unit
    }
}

But how i can know i need to adopt the Equatable?

I tried to remove that protocol and the function works the same. No warnings or errors be reported. Why?

Answer 1

Quoting Apple documentation:

To adopt the Equatable protocol, implement the equal-to operator (==) as a static method of your type

so implementing Equatable means you must overload the == operator, hence this is a build error:

extension StreetAddress: Equatable {
}

Overloading the == operator doesn't require and is not strictly related to Equatable, eg:

class StreetAddress {
    var theAddress:String?

    static func == (lhs: StreetAddress, rhs: StreetAddress) -> Bool {
        return lhs.theAddress?.lowercased() == rhs.theAddress?.lowercased()
    }
}