[Bug libstdc++/56383] error with multiple enable_shared_from_this base classes

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56383

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
On trunk it's now also possible to mix std::enable_shared_from_this and
std::__enable_shared_from_this bases without a hard error:

struct A : std::enable_shared_from_this<A>
{
    void* a() { return shared_from_this().get(); }
};

struct B : std::__enable_shared_from_this<B>
{
};