Optional

gistfile1.txt

https://github.com/apple/swift/blob/master/stdlib/public/core/Optional.swift

public enum Optional<Wrapped> : ExpressibleByNilLiteral {
    // The compiler has special knowledge of Optional<Wrapped>, including the fact
    // that it is an `enum` with cases named `none` and `some`.
    /// The absence of a value.
    ///
    /// In code, the absence of a value is typically written using the `nil`
    /// literal rather than the explicit `.none` enumeration case.
    case none
    
    /// The presence of a value, stored as `Wrapped`.
    case some(Wrapped)
    
    /// Creates an instance that stores the given value.
    @_transparent
    public init(_ some: Wrapped) { self = .some(some) }
    
    /// Creates an instance initialized with `nil`.
    ///
    /// Do not call this initializer directly. It is used by the compiler when you
    /// initialize an `Optional` instance with a `nil` literal. For example:
    ///
    ///     var i: Index? = nil
    ///
    /// In this example, the assignment to the `i` variable calls this
    /// initializer behind the scenes.
    @_transparent
    public init(nilLiteral: ()) {
        self = .none
    }
}