[v3 PATCH] Implement LWG 2842, in_place_t check for optional::optional(U&&) should decay U.

[Jonathan Wakely]

On 19/12/16 16:07 +0200, Ville Voutilainen wrote:
>Tested on Linux-x64.
>
>The perfect forwarder needs to sfinae out of the way of the in_place_t
>signature,
>and the in_place_t signature needs to check is_constructible. I also
>did some housekeeping
>to get rid of the int-pack constraints, because in case of empty packs
>it's unclear
>what a compiler is supposed to do for them.

OK for trunk.