ExpressibleByStringLiteral

Question

为什么（让key1：Keyword =“if”）顺利但（让key2：Keyword = String（“if”））出错？

public enum Keyword: String, ExpressibleByStringLiteral, ExpressibleByUnicodeScalarLiteral, ExpressibleByExtendedGraphemeClusterLiteral {    
case `if` = "if"
case `else` = "else"
public init(stringLiteral:String){
    self.init(rawValue: stringLiteral)!
}
public init(unicodeScalarLiteral value: String) {
    self.init(rawValue: value)!
}
public init(extendedGraphemeClusterLiteral value: String) {
    self.init(rawValue: value)!
}
}

let key1:Keyword = "if" //going well
let key2:Keyword = String("if") //error reported