Hi

Here is my proposal to add support for fancy allocator pointer.

The only place where we still have C pointers is at the 
iterator::pointer level but it's consistent with std::list 
implementation and also logical considering that we do not get 
value_type pointers from the allocator.

I also wondered if it was ok to use nullptr in different places or if I 
should rather do __node_ptr{}. But recent modifications are using 
nullptr so I think it's fine.

     libstdc++: Use allocator::pointer in hashtable implementation

     In _Hashtable implementation store the allocator::pointer returned 
by the allocate
     call as-is and return it on the deallocate when necessary. This is 
true for both
     the allocated nodes and the bucket array.

     libstdc++-v3/ChangeLog:

             * include/bits/hashtable_policy.h: Include 
<bits/stl_uninitialized.h>.
             (__alloc_val_ptr<>): New template alias.
             (_Hash_pnode_base<>): New.
             (_Hash_node<>::__node_ptr): New.
             (_Hash_node<>::__node_type): New.
             (_Hash_pnode<typename _ValuePtr, bool _Cache_hash_code>): New.
             (__get_node_type<>): New, template alias to _Hash_node<> if 
allocator pointer
             type is a native pointer, _Hash_pnode<> otherwise.
             (_Hashtable_iterator_base<typename _NodePtr>): New.
             (_Node_iterator_base<>): Inherits from latter.
             (_Hashtable_iterator<typename _NodePtr, bool 
__constant_iterators>):
             New.
             (_Hashtable_const_iterator<typename _NodePtr, bool 
__constant_iterators>):
             New.
             (_Insert_base<>::__alloc_ptr): New.
             (_Insert_base<>::__hashtable_alloc): Remove.
             (_Insert_base<>::__node_type): New.
             (_Insert_base<>::__node_ptr): New.
             (_Insert_base<>::iterator): Define conditionally to 
_Node_iterator<>
             or _Hashtable_iterator<> depending on __alloc_ptr being a 
raw pointer.
             (_Insert_base<>::const_iterator): Define conditionally to
             _Node_const_iterator<> or _Hashtable_const_iterator<> 
depending on
             __alloc_ptr being a raw pointer.
             (_Hashtable_local_iter_base<>): New.
             (_Hashtable_local_iterator<>): New.
             (_Hashtable_const_local_iterator<>): New.
             (__local_iterator<>): New template alias.
             (__const_local_iterator<>): New template alias.
             (_Hashtable_alloc<>::__get_node_base): New struct.
             (_Hashtable_alloc<>::__value_alloc_traits): Remove.
             (_Hashtable_alloc<>::__node_ptr): Define as 
__node_alloc_traits::pointer.
             (_Hashtable_alloc<>::__node_base): Define as 
__get_node_base<>::pointer.
             (_Hashtable_alloc<>::__node_base_ptr): Define as
             __ptr_rebind<__node_ptr, __node_base>.
             * include/bits/hashtable.h (_Hashtable<>): Adapt.
             * 
testsuite/23_containers/unordered_map/allocator/ext_ptr.cc: New test.
             * 
testsuite/23_containers/unordered_multimap/allocator/ext_ptr.cc:
             New test.
             * 
testsuite/23_containers/unordered_multiset/allocator/ext_ptr.cc:
             New test.
             * 
testsuite/23_containers/unordered_set/allocator/ext_ptr.cc: Adapt.

Tested under Linux x64, ok to commit ?

François