Is there a way to set a typealias of a protocol's associated type in a sub protocol?-CodePudding

I didn't see this in the similar questions, so I think this is unique. I have a protocol

protocol Zot {}

protocol Foo {
    associatedType Bar: Zot
    var prop1: Bar { get set }
}

Now I could say

class Zoty: Zot {}

class Fooy: Foo {
    typealias Bar = Zoty
    var prop1: Zoty = Zoty()
}

But I haven't quite moved to the class layer yet. Is there a way for me to set the type alias in a protocol like so?

protocol Fooie: Foo {
    typealias Bar = Zoty
    var prop1: Zoty { get set }
}

The compiler says to make it like this

protocol Fooie: Foo where Bar == Zoty {
    var prop1: Bar { get set }
}

But then it throws the error Cannot override mutable property 'prop1' of type 'Self.Bar' with covariant type 'Zoty'

CodePudding user response：

To fix the error, just remove this line in Fooie

var prop1: Bar { get set }

You do not need to redeclare this again, and I do agree the error message is a bit confusing. Swift thinks that you are trying to override the property, which as far as it is concerned, is declared to be a different type, but in this context, the two types can only be the same.

After removing the property, you can now do:

class Fooy: Fooie {
    var prop1 = Zoty()
}

and this gives errors

class SomethingElse: Zot {}

class Fooy: Fooie {
    var prop1 = SomethingElse()
}

'Fooie' requires the types 'SomethingElse' and 'Zoty' be equivalent

Type 'Fooy' does not conform to protocol 'Fooie'