Constructing new values in a public function using initializers from an internal protocol

ConstructorMagic.swift

// Using Swift 3.0.1
// see also http://stackoverflow.com/q/42514600/539599

import Foundation

public protocol P {
    var foo: Int { get }
}

internal protocol Q {
    init(from: MyDataRepresentation)
}

internal typealias MyDataRepresentation = [String]

public struct S: P, Q {
    public var foo: Int = 0

    public init() {}

    init(from: MyDataRepresentation) {
        precondition(from.count > 0)
        self.foo = Int(from[0])!
    }
}

public struct T: P, Q {
    public var foo: Int = 77

    public init() {}

    init(from: MyDataRepresentation) {
        precondition(from.count > 0)
        self.foo = Int(from[0])! * 2
    }
}

// A renegade struct, maybe declared by outsiders
public struct U: P {
    public var foo = 99
}

public class ProviderOfThings {
    // Note how the methods do not (and may not!) expose internal API!
  
    public static func map<T: P>(before: T) -> T? {
        guard let beforeQ = before as? Q else {
            return nil
        }
        
        return type(of: beforeQ).init(from: [String(before.foo + 1)]) as! T
    }

    public static func make<T: P>() -> T? {
        guard T.self is Q.Type else {
            return nil
        }

        return (T.self as! Q.Type).init(from: ["0"]) as! T
    }
}

main.swift

print(ProviderOfThings.map(before: myS))
  // Expected: 1
print(ProviderOfThings.map(before: myT))
  // Expected: 156
print(ProviderOfThings.map(before: myU))
  // Expected: nil

let anotherS: S? = ProviderOfThings.make()
let anotherT: T? = ProviderOfThings.make()
let anotherU: U? = ProviderOfThings.make()
print("\(anotherS) -- \(anotherT) -- \(anotherU)")
  // Expected: 1 -- 2 -- nil