TypeInference.swift

enum DogCow: Int {
    case dog, cow
}

struct Wrapper<Value> {
    
    func newValue(from value: Any?) -> Value? {
        
        return value as? Value ?? Optional<Any>.none as? Value
    }
    
    
    func newValue(from value: Any?) -> Value? where Value: RawRepresentable {
        
        return (value as? Value.RawValue).flatMap(Value.init)
    }
}

let int = Wrapper<Int>()
let dogCow = Wrapper<DogCow>()

// what I want
int.newValue(from: 0)     // -> 0
dogCow.newValue(from: 0)  // -> DogCow.dog


func foo<Value>(_ wrapper: Wrapper<Value>, value: Any, type: Any.Type? = nil) -> Value? {
    
    return wrapper.newValue(from: value)
}

let bar = type(of: dogCow)

foo(dogCow, value: 0)  // -> nil