#98 breaks assignment in Boost.Fusion

[Jannik2099]

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)
      |         ^

[pdimov]

@ecatmur

[Jannik2099]

Aiui the original issue is due to llvm/llvm-project#59966 which won't be fixed anytime soon.

I guess the simplest solution is to just define the copy operators aswell - however, I wonder if that could be an API change in itself? For example, do you know if boost or any other big consumer has stuff like has_defaulted_operator<T> ?

Considering that you got no such complaint when the move ctor got added, I would guess no.

[pdimov]

I guess the simplest solution is to just define the copy operators aswell

Not sure what you mean here; do you mean to #ifdef out operator=(T const&) as well, in addition to operator=(T&&)?

If so, you could do a PR with this change and we'll see what CI says about it.