How is Self in protocol interpreted in subclass?

After reading an discussion about using Self in protocol, I did an experiment (see code below). I thought the code would fail to compile because, from my understanding, for class B to conform to Copyable protocol, it should have init(_ instance: B), which I didn't define. But the code actually works.

I wonder why? Thanks for any explanation.

 1  import Foundation
   
 2  protocol Copyable: class {
 3      init(_ instance: Self)
 4  }
   
 5  class A: Copyable {
 6      var x: Int
 7      
 8      init(x: Int) {
 9          self.x = x
10      }
11      
12      required init(_ instance: A) {
13          self.x = instance.x
14      }
15  }
   
16  class B: A {
17      var y: Int
18      
19      init(x: Int, y: Int) {
20          self.y = y
21          super.init(x: x)
22      }
23      
24      required init(_ instance: A) {
25          self.y = 1
26          super.init(instance)
27      }
28  }
   
29  var a = A(x:1)
30  var b = B(a)