Contract.kt

@ExperimentalContracts
fun notNull(reference: Any?) { 
    contract { 
        returns() implies (reference != null) 
    } 
    if (reference == null) throw NullPointerException()
}

/* Now you can use the methid as follows: */

fun logUserInfo(user: User?) { 
    notNull(user)
    
    logger.log(user.id)   // Compiler knows `user` is not null here
}