* include/std/optional (Optional_base(const _Tp&))
(Optional_base(_Tp&&), using _Base::_Base): Remove.
(optional(nullopt_t)): New.
(optional(_Up&&)): Invoke base directly with in_place
rather than creating a temporary, add default template
argument, change constraints.
(optional(const optional<_Up>&)): Invoke base directly
with in_place, remove unnecessary constraints.
(optional(optional<_Up>&& __t)): Likewise.
(optional(in_place_t, _Args&&...)): New.
(optional(in_place_t, initializer_list<_Up>, _Args&&...)): Likewise.
(operator=(_Up&&)): Add default template argument, change constraints.
(operator=(const optional<_Up>&)): Put is_same first in the
constraints.
(operator=(optional<_Up>&&)): Likewise.
* testsuite/20_util/optional/assignment/5.cc: Add a test to
verify assignment from something that can't be perfect-forwarded.
* testsuite/20_util/optional/cons/value.cc: Add tests to verify
that a nested optional is disengaged when constructed
from a disengaged element type, and to verify that assignments
from an engaged element type engage the optional.
From-SVN: r239593
a577f786b6