#98 breaks assignment in Boost.Fusion

Bắt đầu với boost/variant/variant.hpp quanh dòng 620 và boost/fusion/container/deque/deque.hpp quanh dòng 121, sau đó tái hiện lỗi phép gán Boost.Fusion deque đã được báo cáo bằng case Godbolt được liên kết. So sánh hành vi với định nghĩa toán tử điều kiện quanh bản sửa lỗi trong #98; được xem là hoàn thành khi case rút gọn biên dịch được mà không comment đoạn code điều kiện đó.

The fix in #98 breaks usage in boost::fusion::deque.
Commenting out the #if[...]#endif around the operator definition make the code below compile again.

Sadly I encountered this when using Boost.Spirit, so the cvise-reduced case is still quite icky.
Unsure if this is a bug in Boost.Variant, or in how Boost.Fusion uses it.

Godbolt: https://godbolt.org/z/s8EfxT8r4

In file included from <source>:2:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3.hpp:62:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/auxiliary.hpp:11:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/auxiliary/any_parser.hpp:17:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/move_to.hpp:12:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/attribute_category.hpp:16:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/is_variant.hpp:11:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/variant.hpp:17:
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:620:13: error: object of type 'boost::fusion::deque<boost::optional<Slot>, bool>' cannot be assigned because its copy assignment operator is implicitly deleted
  620 |         lhs = rhs_;
      |             ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:1018:16: note: in instantiation of member function 'boost::detail::variant::direct_assigner<boost::fusion::deque<boost::optional<Slot>, bool>>::operator()' requested here
 1018 |         return visitor_(operand);
      |                ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:112:20: note: in instantiation of function template specialization 'boost::detail::variant::invoke_visitor<boost::detail::variant::direct_assigner<boost::fusion::deque<boost::optional<Slot>, bool>>, false>::internal_visit<boost::fusion::deque<boost::optional<Slot>, bool> &>' requested here
  112 |     return visitor.internal_visit(
      |                    ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:152:13: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl_invoke_impl<boost::detail::variant::invoke_visitor<boost::detail::variant::direct_assigner<boost::fusion::deque<boost::optional<Slot>, bool>>, false>, void *, boost::fusion::deque<boost::optional<Slot>, bool>>' requested here
  152 |     return (visitation_impl_invoke_impl)(
      |             ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:235:11: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl_invoke<boost::detail::variant::invoke_visitor<boost::detail::variant::direct_assigner<boost::fusion::deque<boost::optional<Slot>, bool>>, false>, void *, boost::fusion::deque<boost::optional<Slot>, bool>, boost::variant<char, boost::fusion::deque<boost::optional<Slot>, bool>>::has_fallback_type_>' requested here
  235 |         , BOOST_VARIANT_AUX_APPLY_VISITOR_STEP_CASE
      |           ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/repetition/repeat.hpp:30:26: note: expanded from macro 'BOOST_PP_REPEAT'
   30 | # define BOOST_PP_REPEAT BOOST_PP_CAT(BOOST_PP_REPEAT_, BOOST_PP_AUTO_REC(BOOST_PP_REPEAT_P, 4))
      |                          ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/cat.hpp:22:32: note: expanded from macro 'BOOST_PP_CAT'
   22 | #    define BOOST_PP_CAT(a, b) BOOST_PP_CAT_I(a, b)
      |                                ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/cat.hpp:29:34: note: expanded from macro 'BOOST_PP_CAT_I'
   29 | #    define BOOST_PP_CAT_I(a, b) a ## b
      |                                  ^
<scratch space>:148:1: note: expanded from here
  148 | BOOST_PP_REPEAT_1
      | ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:2282:33: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl<mpl_::int_<0>, boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2>, char, boost::mpl::l_item<mpl_::long_<1>, boost::fusion::deque<boost::optional<Slot>, bool>, boost::mpl::l_end>>>, boost::mpl::l_iter<l_end>>, boost::detail::variant::invoke_visitor<boost::detail::variant::direct_assigner<boost::fusion::deque<boost::optional<Slot>, bool>>, false>, void *, boost::variant<char, boost::fusion::deque<boost::optional<Slot>, bool>>::has_fallback_type_>' requested here
 2282 |         return detail::variant::visitation_impl(
      |                                 ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:2294:16: note: (skipping 12 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
 2294 |         return internal_apply_visitor_impl(
      |                ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:209:26: note: in instantiation of function template specialization 'boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>::parse<__gnu_cxx::__normal_iterator<char *, std::basic_string<char>>, boost::spirit::x3::context<boost::spirit::x3::rule<atomSlotExpr>, const boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>>, boost::spirit::x3::unused_type>' requested here
  209 |             bool r = rhs.parse(
      |                          ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:265:20: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser<boost::spirit::x3::rule<atomSlotExpr>, boost::spirit::x3::rule<atomSlotExpr>>::parse_rhs_main<boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>, __gnu_cxx::__normal_iterator<char *, std::basic_string<char>>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, const boost::spirit::x3::unused_type>' requested here
  265 |             return parse_rhs_main(
      |                    ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:290:20: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser<boost::spirit::x3::rule<atomSlotExpr>, boost::spirit::x3::rule<atomSlotExpr>>::parse_rhs_main<boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>, __gnu_cxx::__normal_iterator<char *, std::basic_string<char>>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, const boost::spirit::x3::unused_type>' requested here
  290 |             return parse_rhs_main(rhs, first, last, context, rcontext, unused);
      |                    ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:328:28: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser<boost::spirit::x3::rule<atomSlotExpr>, boost::spirit::x3::rule<atomSlotExpr>>::parse_rhs<boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>, __gnu_cxx::__normal_iterator<char *, std::basic_string<char>>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type>' requested here
  328 |                 ok_parse = parse_rhs(rhs, first, last, context, attr_, attr_
      |                            ^
<source>:19:45: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser<boost::spirit::x3::rule<atomSlotExpr>, boost::spirit::x3::rule<atomSlotExpr>>::call_rule_definition<boost::spirit::x3::action<boost::spirit::x3::alternative<boost::spirit::x3::any_char<boost::spirit::char_encoding::standard>, boost::spirit::x3::sequence<boost::spirit::x3::optional<boost::spirit::x3::rule<slot_struc, Slot>>, boost::spirit::x3::matches_directive<boost::spirit::x3::literal_string<const char *, boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>>>>, (lambda at <source>:9:25)>, __gnu_cxx::__normal_iterator<char *, std::basic_string<char>>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, mpl_::bool_<false>>' requested here
   19 |   x3::detail ::rule_parser<rule_t, rule_t>::call_rule_definition(
      |                                             ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/fusion/container/deque/deque.hpp:121:9: note: copy assignment operator is implicitly deleted because 'deque<boost::optional<Slot>, bool>' has a user-declared move constructor
  121 |         deque(deque&& seq)
      |         ^