Protocol covariance

Protocol-covariance.swift

// Covariance works properly with classes:

class MyType {}
class MySubtype: MyType {}
class MyClass {
    func createMyType() -> MyType {
        return MyType()
    }
}

class MySubClass: MyClass {
    override func createMyType() -> MySubtype {
        return MySubtype()
    }
}

// However it doesn't work for protocol conformance:

protocol MyProtocol {
    func createMyType() -> MyType
}

class MyConformingClass: MyProtocol {
    func createMyType() -> MySubtype {
        return MySubtype()
    }
}

//Compiler error for this case:
//error: type 'MyConformingClass' does not conform to protocol 'MyProtocol'
//note: protocol requires function 'createMyType()' with type '() -> MyType'
//note: candidate has non-matching type '() -> MySubtype'

protocol MyProtocol {
    associatedtype SomeType
    func createMyType() -> SomeType
}

class MyConformingClass: MyProtocol {
    typealias SomeType = MySubtype
    func createMyType() -> MySubtype {
        return MySubtype()
    }
}