[Bug libstdc++/90008] New: [9 Regression] variant attempts to copy rhs in comparison operators

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90008

            Bug ID: 90008
           Summary: [9 Regression] variant attempts to copy rhs in
                    comparison operators
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

_VARIANT_RELATION_FUNCTION_TEMPLATE accidentally accepts the second visitable
by copy in `__do_visit<__detail::__variant::__visit_with_index>`. 

The following test fails right now, but worked in GCC-8:

#include <variant>

struct user_defined {
    user_defined();
    user_defined(const user_defined&) = delete;
    user_defined(user_defined&&) = delete;
};

bool operator==(const user_defined& x, const user_defined& y) { return true; }

using v_t = std::variant<user_defined, int>;

auto test(const v_t& v, const v_t& v2) {
    return v == v2;
}