Q:

Generic of type RawRepresentable is misinterpreted as self it seems

To use NSCoding with Swift's Enum type I made an extension on NSCoder:
extension NSCoder {
    func encodeEnum<Enum: RawRepresentable where Enum.RawValue == String>(value: Enum, forKey key: String) {
        self.encodeObject(value.rawValue, forKey: key)
    }

    func decodeEnumForKey<Enum: RawRepresentable where Enum.RawValue == String>(key: String) -> Enum? {
        guard let returnValue = self.decodeObjectForKey(key) as? String else { return nil }
        return Enum(rawValue: returnValue)
    }
}

The encodeEnum method works fine for a String-backed Enum, but when I try to decode the prior encoded Enum like so:
enum MyEnum: String { case Something, Other }
class MyEnumClass: NSObject, NSCoding {
    let myEnum: MyEnum

    init(myEnum: MyEnum) {
        self.myEnum = myEnum
    }

    required convenience init?(coder aDecoder: NSCoder) {
        guard let tmp = aDecoder.decodeEnumForKey("myKey") as? MyEnum else { return nil }

        self.init(myEnum: tmp)
    }
}

I get an error on aDecoder.decodeEnumForKey("myKey"):
Value of type `NSCoder` has no member `RawValue`

I'm pretty sure it has something to do with the generic and the condition that Enum.RawValue == String. But I do not understand while it's not working, but works for encodeEnum().

A:

The problem is that in 
guard let tmp = aDecoder.decodeEnumForKey("myKey") as? MyEnum else { return nil }

the compiler cannot infer the generic placeholder of 
func decodeEnumForKey<Enum: ...>(key: String) -> Enum?

to be MyEnum, you have to cast the result to MyEnum? instead:
guard let tmp = aDecoder.decodeEnumForKey("myKey") as MyEnum? else { return nil }

so that the return type is inferred as MyEnum? from the calling context.

