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

internal protocol Q {
    init(from: [String])
}

public struct S: P, Q {
    public var foo: Int = 0
    public init() {}

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

public class ProviderOfThings {
    public func map<T: P>(before: T) -> T {
        return T(from: [String(before.foo + 1)])
    }
}

public static func map<T: P>(before: T) -> T {
    return type(of: before as! Q).init(
        from: [String(before.foo + 1)]
    ) as! T
}