Allow dynamic keyword on non-objc properties

Evolution Pitches
7

Yesterday I tried whether I can use dynamic properties with String-based enums (does not work). If the following code would work a lot of boilerplate could be avoided for a exact set of functions known at compile-time:

class Token {
    
    enum DoubleGroup: String { case expiresAfter, ... }
    enum StringGroup: String { case issuer, ... }
    
    subscript(dynamicMember member: DoubleGroup) -> Double? { ... }
    subscript(dynamicMember member: StringGroup) -> String? { ... }
}

Of course the type checker should be able to disambiguate the following code:

let exp = token.expiresAfter // -> Double?
let iss = token.issuer // -> String?