This is PR 96088 but this time for _Rb_tree based containers.

I guess it won't go in for the moment but I wanted to submit it already 
because of the changes I had to do in stl_functions.h. It sounds like 
missing parts for C++11 move-semantic. I still need to run all tests to 
see if they can have side effects.

      libstdc++: [_Rb_tree] Limit allocation on iterator insertion [PR 
96088]

     Detect when invoking the comparer require an allocation and in this 
case
     create a temporary instance that will be moved to storage location 
if the
     insertion eventually takes place. Avoid to allocate a node otherwise.

     libstdc++-v3/ChangeLog:

             PR libstdc++/96088
             * include/bits/stl_function.h
             (std::less<>::operator()): Add noexcept qualification.
             (std::greater::operator()): Likewise.
(std::_Identity<>::operator<_Tp2>(_Tp2&&)): New perfect forwarding operator.
(std::_Select1st<>::operator<_Pair2>(_Pair2&&)): New move operator.
             * include/bits/stl_tree.h 
(_Rb_tree<>::_ConvertToValueType<>): New helper type.
             (_Rb_tree<>::_M_get_insert_unique_pos_tr): New.
             (_Rb_tree<>::_S_forward_key): New.
             (_Rb_tree<>::_M_emplace_unique_kv): New.
             (_Rb_tree<>::_M_emplace_unique_aux): New, use latter.
             (_Rb_tree<>::_M_emplace_unique): New, use latter.
             * testsuite/23_containers/map/96088.cc: New test case.
             * testsuite/23_containers/multimap/96088.cc: New test case.
             * testsuite/23_containers/multiset/96088.cc: New test case.
             * testsuite/23_containers/set/96088.cc: New test case.

Ok to commit ?

François