Decodable conformance with other protocols in Swift

Question

The issue is when a struct conforms to a protocol (let's call it PA) and Decodable, but PA imposes a property with a type that is not Decodable. Example:

protocol PA {
    var b: [PB]? { get }
}

protocol PB {}

struct SA: PA, Decodable {
    let b: [PB]? // SA's conformance to Decodable wants this to be [Decodable], but PA's conformance imposes [PB]
}

struct SB: PB, Decodable {}

the code above refuses to compile, with:

• error: type 'SA' does not conform to protocol 'Decodable'
• note: cannot automatically synthesize 'Decodable' because '[PB]?' does not conform to 'Decodable'

Changing that line to:

let b: [PB & Decodable]?

does not work either and gives:

• error: type 'SA' does not conform to protocol 'PA'
• note: candidate has non-matching type '[Decodable & PB]?'
• error: type 'SA' does not conform to protocol 'Decodable'
• note: cannot automatically synthesize 'Decodable' because '[Decodable & PB]?' does not conform to 'Decodable'
• note: protocol requires property 'b' with type '[PB]?'; do you want to add a stub?

Note that the 4th line is non-sense: "'[Decodable & PB]?' does not conform to 'Decodable'". Wait what?

Any suggestion?

Answer 1

You may create a mixed protocol:

protocol PADecodable {
    var b: [PB & Decodable]? { get }
}

struct SA: PADecodable {
    let b: [PB & Decodable]?
}

Answer 2

you can fix it by:

protocol PA {
    var b: [PB]? { get }
}

protocol PB {}

struct SA<T: PB & Codable>: PA, Codable {
    private var _b: [T]?
    
    var b: [PB]? {
        return _b
    }
}