When inserting into unordered_multiset or unordered_multimap first instantiate
the node to store and compute the hash code from it to avoid a potential
intermediate key_type instantiation.
When inserting into unordered_set or unordered_map check if invoking the hash
functor with container key_type is noexcept and invoking the same hash functor
with key part of the iterator value_type can throw. In this case create a
temporary key_type instance at Hashtable level and use it to compute the hash
code. This temporary instance is moved to the final storage location if needed.
libstdc++-v3/ChangeLog:
PR libstdc++/96088
* include/bits/hashtable_policy.h (_Select2nd): New.
(_NodeBuilder<>): New.
(_ReuseOrAllocNode<>::operator()): Use variadic template args.
(_AllocNode<>::operator()): Likewise.
* include/bits/hashtable.h
(_Hashtable<>::__node_builder_t): New.
(_Hashtable<>::_M_insert_unique<>(_Kt&&, _Arg&&, const _NodeGenerator&)):
New.
(_Hashtable<>::_S_forward_key): New.
(_Hashtable<>::_M_insert): Use latter.
(_Hashtable<>::_M_insert(const_iterator, _Arg&&, const _NodeGenerator&, false_type)):
Instantiate node first, compute hash code second.
* testsuite/23_containers/unordered_map/96088.cc: New test.
* testsuite/23_containers/unordered_multimap/96088.cc: New test.
* testsuite/23_containers/unordered_multiset/96088.cc: New test.
* testsuite/23_containers/unordered_set/96088.cc: New test.
* testsuite/util/replacement_memory_operators.h
(counter::_M_increment): New.
(counter::_M_decrement): New.
(counter::reset()): New.
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