N3644 implies that operator- can be used on value-init iterators. We now return
0 if both iterators are value initialized. If only one is value initialized we
keep the UB by returning the result of a normal computation which is a meaningless
value.
libstdc++-v3/ChangeLog:
PR libstdc++/70303
* include/bits/stl_deque.h (std::deque<>::operator-(iterator, iterator)):
Return 0 if both iterators are value-initialized.
* testsuite/23_containers/deque/70303.cc: New test.
* testsuite/23_containers/vector/70303.cc: New test.
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