Re: input iterators and post increment

I'm using input iterators to implement an asynchronous IO system.

Whenever you increment an input iterator, all copies of the previous
value can become invalidated. Indeed, this is the case in my example as
blocks of data come in and are released back to the system as the
iterator marches forward.

So my question in all this is why does the input iterator require post
increment?
It seems like an inherently broken and unsafe operation to support,
given that the only feature proffered by the useless return value is
the opportunity to write bugs by accidentally dereferencing it.

Would it break STL algorithms or in some other way be incorrect to only
implement pre-increment?

What about this variant?

void operator++(int);

Or just do the same as pre-increment?

iterator& operator++(int) { return ++(*this); }

If it must be implemented, perhaps the best practice is instead of returning
a copy of the previous iterator, return an iterator with some sentinel
values like setting all of its internal pointer data members to nullptr and
then asserting on that in the dereference.