Code contracts

As smart as compilers are these days, there are scenarios that don't have enough context and yield compilation errors. However, those errors cannot possibly occur here. You might have come across functions similar, at least logically, to the following one:

data class Command(val timestamp:Long)

fun processCommand(command:Command?){
  validate(command)
  println(command.timestamp)
}

fun validate(command:Command?){
  if(command == null) {
    throw new IllegalArgumentException("Invalid 'command' parameter. Expecting non-null parameter")
  }

  //... more validation here
}

If you were to compile this code as it is, the compiler will return an error at println(command.type). It will say—Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Command?. Given the earlier validate method, we know the error can never occur.
Wouldn't it be nice if we have a way to inform the compiler that the validate function already checks for null and, therefore, avoid the compilation error? Since Kotlin 1.3, you can do exactly this through the use of the Contracts API.