Class does not conform to protocol, but struct does

Question

Both a struct and a class conforms to a protocol. I use 2 protocol extensions with where conditions to add an implementation of the var property for both class and struct.

I'm quite surprised to see a compile error for classes only.

Why does this happen for classes and not for structs?

protocol MyProtocol {
    var property:String { get }
}

extension MyProtocol where Self == MyStruct {
    var property: String { return "" }
}

extension MyProtocol where Self == MyClass {
    var property: String { return "" }
}

struct MyStruct : MyProtocol {}

class MyClass : MyProtocol {} //Type 'MyClass' does not conform to protocol 'MyProtocol'

Answer 1

It does not compile because your

extension MyProtocol where Self == MyClass

provides a default method only for MyClass itself, but not for possible subclasses. Changing the constraint to

extension MyProtocol where Self: MyClass

makes the code compile. Alternatively prevent the creation of subclasses with

final class MyClass : MyProtocol {}

This is not a problem for MyStruct because struct types cannot be inherited from in Swift.

Answer 2

Classes can inherit from other classes.

So some class X may inherit from MyClass .

But your extension provides implementation of MyProtocol properties of if the class is MyClass and not it's descendant. So any class that inherits from MyClass won't have any properties of MyProtocol implemented. And that is the problem.

If you declare MyClass final your code would be valid:

final class MyClass : MyProtocol {}

If you extend conditional extension to any MyClass descendants your code would be valid:

extension MyProtocol where Self: MyClass {
    var property: String { return "" }
}

But right now all those potential subclasses lack implementation for MyProtocol and it is not allowed.