How to define initializers in a protocol extension?

Question

protocol Car {
     var wheels : Int { get set}

     init(wheels: Int)

}

extension Car {
    init(wheels: Int) {
        self.wheels = wheels
    }
}

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Answer 1

My understanding is that this isn't possible, because the protocol extension can't know which properties the conforming class or struct has - and therefore cannot guarantee they are correctly initialized.

If there are ways to get around this, I'm very interested to know! :)