Defaulting the copy constructor on its first declaration made it change
from user-provided (and non-trivial) to implicitly-defined (and
trivial). This caused an ABI incompatibility between GCC 8 and GCC 9,
where functions taking a deque iterator disagree on the argument passing
convention.
PR libstdc++/92267
* include/bits/stl_deque.h (_Deque_iterator(const _Deque_iterator&)):
Do not define as defaulted.
* testsuite/23_containers/deque/types/92267.cc: New test.
From-SVN: r277577
