The standard states at 23.2.1.3/4:

deque erase(...)

Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.

This does not state that iterators to end will be invalidated.
It needs to be amended in such a way as to account for end invalidation.

Something like:
Any time the last element is erased, iterators to end are invalidated.

This would handle situations like:
erase(begin(), end())
erase(end() - 1)
pop_back()
resize(n, ...) where n < size()

pop_front() with size() == 1

   -Steve




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