Type any <Protocol> cannot conform to Hashable

Question

I'm playing with the Swift 5.7 new capabilities and I'm trying to implement some heterogeneous Sets.

I've reduced my code to this simple playground:

protocol Stuff: Hashable {
    var identifier: String { get }
}

struct StuffA: Stuff {
    let identifier: String
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(identifier)
    }
}

struct StuffB: Stuff {
    let identifier: String
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(identifier)
    }
}

var arrayOfStuff: Array<any Stuff> = []
arrayOfStuff.append(StuffA(identifier: "a"))
arrayOfStuff.append(StuffB(identifier: "b"))
// Works like a charm

var setOfStuff: Set<any Stuff> = Set() 
// error: Type 'any Stuff' cannot conform to 'Hashable'
setOfStuff.insert(StuffA(identifier: "a"))
setOfStuff.insert(StuffB(identifier: "b"))

The first try with Array<any Stuff> shows any can be used with a protocol to specify a generic constraint.

The second try fails because swift cannot checks the Hashable conformance. I know that Stuff does not conforms to Hashable, it only inherits from. But how do you tell the compiler that any Stuff will conform?

Answer 1

The issue is that Set requires a concrete type not an "any". This is a frustrating part of swift for now. You would have to make a type that conforms to the protocol, but still explicitly make it conform to a separate requirement/protocol?. I've recently run into the same situation in complex code and it's a real bummer. Basically you would have to make an arbitrary type that conforms to your protocol and the required "Hashable" In my case I made a LineItem wrapper that wraps a "any LineItemRepresentable(my protocol)" and conforms to the requirement that I couldn't just support by requiring the protocol to support it..

here's an example

protocol SomeProtocol {
    var item {get set}
}
    
class MyClass: Hashable {
    var wrappedValue: any SomeProtocol
    
    // init...
    
    var item: Item {
       get {
          wrappedValue.item
       }
       set {
          wrappedValue.item = newValue
       }
    }
 }