[v3 PATCH] Implement LWG 2451

[Jonathan Wakely]

On 06/07/16 00:44 +0300, Ville Voutilainen wrote:
>    Implement LWG 2451, optional<T> should 'forward' T's
>    implicit conversions.
>    * include/experimental/optional (__is_optional_impl, __is_optional):
>    New.
>    (optional()): Make constexpr and default.
>    (optional(_Up&&), optional(const optional<_Up>&),
>    optional(optional<_Up>&& __t): New.
>    (operator=(_Up&&)): Constrain.
>    (operator=(const optional<_Up>&), operator=(optional<_Up>&&)): New.
>    * testsuite/experimental/optional/cons/value.cc:
>    Add tests for the functionality added by LWG 2451.
>    * testsuite/experimental/optional/cons/value_neg.cc: New.

As I mentioned on IRC, experimental::optional is C++14-only so you can
use remove_cv_t and enable_if_t to get rid of some of the typename
X::type clutter.

OK for trunk and gcc-6 with or without that change, your choice.