This fixes the remaining problem reported in the PR, that the special
members should be trivial. This can be done by constraining the
non-trivial versions and adding defaulted overloads that will be used
when the union members are trivial.
Making these members trivial alters the argument passing ABI and so
isn't suitable for backporting to release branches.
libstdc++-v3/ChangeLog:
PR libstdc++/100823
* include/bits/stl_iterator.h (common_iterator): Define
destructor, copy constructor and move constructor as trivial
when the underlying types allow.
* testsuite/24_iterators/common_iterator/100823.cc: Check
triviality of special members.
87a9bfe86d