Undefined Func in Swift

Undefined.swift

///
/// From: https://realm.io/news/swift-summit-johannes-weiss-the-type-system-is-your-friend/
///
/// Slightly improved:
///     * Report function, file and line of actual call site
///     * Warn usage of this function (somewhat hacky though)
///     * Discardable result
@available(*, deprecated, message: "Use of undefined()")
@discardableResult public func undefined<T>(
    _ message: StaticString = ""
    , function: StaticString = #function
    , file: StaticString = #file
    , line: UInt = #line)
    -> T {
        fatalError(
            "undefined(\(message)) in \(function)",
            file: file,
            line : line)
}