Better `fatalError()`.

FatalError+Ext.swift

import Foundation

func fatalError<T>(@autoclosure message: () -> String = "", file: StaticString = #file, line: UInt = #line) -> T {
    fatalError(message(), file: file, line: line)
}

// Demo

protocol ResultType {

    associatedtype V

    var value: V? { get }
    var error: ErrorType? { get }

}

enum Result<V>: ResultType {

    case Value(V)
    case Error(ErrorType)

    var value: V? {
        switch self {
            case let .Value(value):
                return value
            default:
                return nil
        }
    }

    var error: ErrorType? {
        switch self {
            case let .Error(error):
                return error
            default:
                return nil
        }
    }

    init(value: V) {
        self = .Value(value)
    }

    init<R: ResultType where R.V == V>(sourceResult r: R) {
        self = r.value.map { .Value($0) } ?? r.error.map { .Error($0) } ?? fatalError() // LOOK HERE
    }

}

struct InvalidResult<V>: ResultType {

    var value: V? { return nil }
    var error: ErrorType? { return nil }

}

print(Result(Result.Value("OK")))
print(Result(InvalidResult<()>()))

/*
Value("OK")
fatal error: : file /var/folders/mz/4rdq8yg95v56bhbndfvnr7z4lwl1px/T/lldb/70553/playground36.swift, line 40
*/