[v3 PATCH] Do the operator= SFINAE in the return type for optional, not in the template parameters.

[Jonathan Wakely]

On 19/10/16 22:35 +0300, Ville Voutilainen wrote:
>    Do the operator= SFINAE in the return type for optional,
>    not in the template parameters.
>    * include/std/optional (operator=(_Up&&)): Move SFINAE
>    from template parameters to the return type.
>    (operator=(const optional<_Up>&)): Likewise.
>    (operator=(optional<_Up>&&)): Likewise.

OK.