[Bug libstdc++/56785] std::tuple of two elements does not apply empty base class optimization when one of its elements is a std::tuple with two elements

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56785

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The cause is that every N-tuple hierarchy ends with an empty _Tuple_impl<N>
base, so when there are several of those in the same hierarchy they need to
have separate addresses.

It should be possible to fix by eliminating that terminal objet in the
inheritance hierarchy. The obvious way duplicates a chunk of code, I think it
should be possible to do it in a nicer way, which I'm experimenting with.