Following confirmation of the fix by TC here is the patch where I'm 
simply adding a 'constexpr' on _M_next().

Please let me know this ChangeLog entry is correct. I would prefer this 
patch to be assigned to 'TC' with me as co-author but I don't know how 
to do such a thing. Unless I need to change my user git identity to do so ?

     libstdc++: Add constexpr qualification to _Hash_node::_M_next()

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=1b6f0476837205932613ddb2b3429a55c26c409d
     changed _Hash_node_value_base to no longer derive from 
_Hash_node_base, which means
     that its member functions expect _M_storage to be at a different 
offset. So explosions
     result if an out-of-line definition is emitted for any of the 
member functions (say,
     in a non-optimized build) and the resulting object file is then 
linked with code built
     using older version of GCC/libstdc++.

     libstdc++-v3/ChangeLog:

             * include/bits/hashtable_policy.h
             (_Hash_node_value_base<>::_M_valptr(), 
_Hash_node_value_base<>::_M_v())
             Add [[__gnu__::__always_inline__]].
             (_Hash_node<>::_M_next()): Add constexpr.

     Co-authored-by: TC <rs2740@gmail.com>

Ok to commit and backport to GCC 11, 12, 13 branches ?

François