[Bug libstdc++/51365] cannot use final empty class in std::tuple

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51365

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-11 13:39:20 UTC ---
I haven't really thought about it either, but one advantage of depending on
is_empty is that it prevents using the EBO for polymorphic classes, where
derivation could have undesirable effects.  That *shouldn't* be a problem as
long as user-defined types don't have virtual functions using reserved names
such as _M_get.

Another problem could be user-defined types with a virtual base, if the tuple
node type (which would be the most-derived type) doesn't know how to construct
the virtual base.

struct A { A(int) { } };

struct B : virtual A { B() : A(1) { } };

std::tuple<B> t;

If tuple derived from B it couldn't construct the A subobject correctly.