Casting away const and invariant

[Lutger]

Janice Caron wrote:
>> It may well be necessary; if it is, the behaviour must be defined
>> so that it _works_.
> 
> No, it's undefined for a reason. It should stay undefined.

Just to be clear: I assume the behavior of casting away const and then 
reading the ex-const variable is defined, otherwise it doesn't make much 
sense. The only thing undefined is writing a const casted variable.