[PATCH] PR libstdc++/90388 fix std::hash<unique_ptr<T,D>> bugs

[Ville Voutilainen]

On Sat, 11 May 2019 at 00:55, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 11/05/19 00:51 +0300, Ville Voutilainen wrote:
> >On Sat, 11 May 2019 at 00:42, Jonathan Wakely <jwakely@redhat.com> wrote:
> >>
> >> A disabled specialization should not be callable, so move the function
> >> call operator into a new base class which correctly implements the
> >> disabled hash semantics. For the versioned namespace configuration do
> >> not derive from __poison_hash in the enabled case, as the empty base
> >> class serves no purpose but potentially increases the object size. For
> >> the default configuration that base class must be kept, to preserve
> >> layout.
> >
> >I continue to not be a fan of the versioned namespace ifdeffery in
> >this, but I can live with it.
>
> The versioned namespace configuration should be as good as we can make
> it, with no limitations due to ABI compatibility. Removing redundant
> base classes allows more compact class layouts. With current trunk
> this type has size 3, in the versioned namespace after my patch it has
> size 2 (and if we patched hash<optional<T>> it would have size 1):

I understand all of that, but I do question the value of optimizing
the versioned namespace configuration at the cost
of sprinkling such ifdefs into our code.