[v3 PATCH] Constrain optional's __constexpr_addressof in its return type, Use a constexpr addressof for optional, if available.

[Jonathan Wakely]

On 06/12/16 10:52 +0200, Ville Voutilainen wrote:
>The background for these is at
>https://stackoverflow.com/questions/40983822/cannot-use-a-constant-reference-of-experimentaloptional-when-using-clang-lib
>
>We use excessive trickery to constrain __constexpr_addressof in
>experimental::optional.
>That trickery is not agreed on by compilers, and we can cope with a much simpler
>approach.
>
>The first patch just simplifies that trickery by moving the
>constraints from a template
>parameter to the return type.
>
>The second patch uses a constexpr std::__addressof if it's available.
>I split these
>out in case we want just the first patch but not the second. The second patch
>allows C++17 users to benefit from a completely constexpr addressof even
>for experimental::optional (std::optional already gives that).
>
>Tested on Linux-x64. Ok for trunk, the gcc-6 branch and the gcc-5 branch?

Yes, both patches are OK for all branches (they only affect
experimental TS material and/or C++17 mode, so OK for stage 3).

Thanks.