Thread: I upgraded to g++ 4.2.2 and...

If x is not a valid iterator, i.e. x has the so-called singular value, then absolutely every operation except for assignment and destruction will invoke undefined behaviour. It is not true that x != y for any valid y, because the comparison invokes undefined behaviour. It is not true that x == x, because the comparison invokes undefined behaviour.

To do anything with an iterator at all, it needs to have a valid value.

24.1/5 says:

Iterators can also have singular values that are not associated with any container. [...] Results of most expressions are undefined for singular values; the only exception is an assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable and past-the-end values are always non-singular.

I believe a later defect report clarified that destruction is a valid operation on a singular iterator.