clang scelta build error

error.txt

In file included from /home/jackie/code/scelta/example/error_handling.cpp:6:
In file included from /home/jackie/code/scelta/example/./example_utils.hpp:8:
In file included from /home/jackie/code/scelta/include/scelta.hpp:10:
In file included from /home/jackie/code/scelta/include/./scelta/support.hpp:8:
In file included from /home/jackie/code/scelta/include/./scelta/./support/variant.hpp:8:
In file included from /home/jackie/code/scelta/include/./scelta/./support/./variant/std.hpp:13:
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:46: error: cannot cast 'std::variant<io_outcome::success, io_outcome::not_found>' to its private base class 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:1233:35: note: in instantiation of function template specialization 'std::__detail::__variant::__get<1, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
      return __detail::__variant::__get<_Np>(std::move(__v));
                                  ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:676:9: note: in instantiation of function template specialization 'std::get<1, io_outcome::success, io_outcome::not_found>' requested here
                        std::get<__indices>(
                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:682:29: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 1> >::__visit_invoke' requested here
      { return _Array_type{&__visit_invoke}; }
                            ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:659:53: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 1> >::_S_apply' requested here
              std::index_sequence<__indices..., __index>>::_S_apply();
                                                           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:647:5: note: in instantiation of function template specialization 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&), 2>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long> >::_S_apply_single_alt<1, std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)> >' requested here
          (_S_apply_single_alt<__var_indices>(
           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:637:2: note: (skipping 4 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        _S_apply_all_alts(
        ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/std.hpp:16:58: note: in instantiation of function template specialization 'std::visit<const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
SCELTA_DEFINE_HOMOGENIZER_VARIANT(::std::variant, ::std::visit)
                                                         ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/../../utils/homogenizer.hpp:36:13: note: while substituting deduced template arguments into function template 'operator()' [with Ts = <const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found>>]
            homogenizer_helper_t<std::decay_t<Variant>>{}(
            ^
/home/jackie/code/scelta/include/./scelta/./recursive/../visitation/./visit.hpp:21:13: note: while substituting deduced template arguments into function template 'visit_homogenizer' [with Visitor = const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            impl::visit_homogenizer(
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:37:13: note: while substituting deduced template arguments into function template 'visit' [with Visitor = const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            DEFINE_BOUND_VISITOR_CALL(const Visitor&, const&)
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:30:13: note: expanded from macro 'DEFINE_BOUND_VISITOR_CALL'
            visit(std::declval<m_visitor_type>(), FWD(variants)...)         \
            ^
/home/jackie/code/scelta/example/error_handling.cpp:56:9: note: while substituting deduced template arguments into function template 'operator()' [with Variants = <std::variant<io_outcome::success, io_outcome::not_found>>]
        (read_file(argv[1]));
        ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:74: error: '_M_u' is a private member of 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                                                         ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:356:34: note: member is declared here
      _Variadic_union<_Types...> _M_u;
                                 ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:46: error: cannot cast 'std::variant<io_outcome::success, io_outcome::not_found>' to its private base class 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:1233:35: note: in instantiation of function template specialization 'std::__detail::__variant::__get<0, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
      return __detail::__variant::__get<_Np>(std::move(__v));
                                  ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:676:9: note: in instantiation of function template specialization 'std::get<0, io_outcome::success, io_outcome::not_found>' requested here
                        std::get<__indices>(
                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:682:29: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 0> >::__visit_invoke' requested here
      { return _Array_type{&__visit_invoke}; }
                            ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:659:53: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 0> >::_S_apply' requested here
              std::index_sequence<__indices..., __index>>::_S_apply();
                                                           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:647:5: note: in instantiation of function template specialization 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&), 2>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long> >::_S_apply_single_alt<0, std::__detail::__variant::_Multi_array<int (*)(const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> &&)> >' requested here
          (_S_apply_single_alt<__var_indices>(
           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:637:2: note: (skipping 4 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        _S_apply_all_alts(
        ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/std.hpp:16:58: note: in instantiation of function template specialization 'std::visit<const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
SCELTA_DEFINE_HOMOGENIZER_VARIANT(::std::variant, ::std::visit)
                                                         ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/../../utils/homogenizer.hpp:36:13: note: while substituting deduced template arguments into function template 'operator()' [with Ts = <const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, std::variant<io_outcome::success, io_outcome::not_found>>]
            homogenizer_helper_t<std::decay_t<Variant>>{}(
            ^
/home/jackie/code/scelta/include/./scelta/./recursive/../visitation/./visit.hpp:21:13: note: while substituting deduced template arguments into function template 'visit_homogenizer' [with Visitor = const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            impl::visit_homogenizer(
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:37:13: note: while substituting deduced template arguments into function template 'visit' [with Visitor = const scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            DEFINE_BOUND_VISITOR_CALL(const Visitor&, const&)
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:30:13: note: expanded from macro 'DEFINE_BOUND_VISITOR_CALL'
            visit(std::declval<m_visitor_type>(), FWD(variants)...)         \
            ^
/home/jackie/code/scelta/example/error_handling.cpp:56:9: note: while substituting deduced template arguments into function template 'operator()' [with Variants = <std::variant<io_outcome::success, io_outcome::not_found>>]
        (read_file(argv[1]));
        ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:74: error: '_M_u' is a private member of 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                                                         ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:356:34: note: member is declared here
      _Variadic_union<_Types...> _M_u;
                                 ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:46: error: cannot cast 'const std::variant<io_outcome::success, io_outcome::not_found>' to its private base class 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:1255:35: note: in instantiation of function template specialization 'std::__detail::__variant::__get<1, const std::variant<io_outcome::success, io_outcome::not_found> >' requested here
      return __detail::__variant::__get<_Np>(std::move(__v));
                                  ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:676:9: note: in instantiation of function template specialization 'std::get<1, io_outcome::success, io_outcome::not_found>' requested here
                        std::get<__indices>(
                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:682:29: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 1> >::__visit_invoke' requested here
      { return _Array_type{&__visit_invoke}; }
                            ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:659:53: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 1> >::_S_apply' requested here
              std::index_sequence<__indices..., __index>>::_S_apply();
                                                           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:647:5: note: in instantiation of function template specialization 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&), 2>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long> >::_S_apply_single_alt<1, std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)> >' requested here
          (_S_apply_single_alt<__var_indices>(
           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:637:2: note: (skipping 4 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        _S_apply_all_alts(
        ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/std.hpp:16:58: note: in instantiation of function template specialization 'std::visit<scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
SCELTA_DEFINE_HOMOGENIZER_VARIANT(::std::variant, ::std::visit)
                                                         ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/../../utils/homogenizer.hpp:36:13: note: while substituting deduced template arguments into function template 'operator()' [with Ts = <scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, std::variant<io_outcome::success, io_outcome::not_found>>]
            homogenizer_helper_t<std::decay_t<Variant>>{}(
            ^
/home/jackie/code/scelta/include/./scelta/./recursive/../visitation/./visit.hpp:21:13: note: while substituting deduced template arguments into function template 'visit_homogenizer' [with Visitor = scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            impl::visit_homogenizer(
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:38:13: note: while substituting deduced template arguments into function template 'visit' [with Visitor = scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            DEFINE_BOUND_VISITOR_CALL(Visitor&&,      &&)
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:30:13: note: expanded from macro 'DEFINE_BOUND_VISITOR_CALL'
            visit(std::declval<m_visitor_type>(), FWD(variants)...)         \
            ^
/home/jackie/code/scelta/example/error_handling.cpp:56:9: note: while substituting deduced template arguments into function template 'operator()' [with Variants = <std::variant<io_outcome::success, io_outcome::not_found>>]
        (read_file(argv[1]));
        ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:74: error: '_M_u' is a private member of 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                                                         ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:356:34: note: member is declared here
      _Variadic_union<_Types...> _M_u;
                                 ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:46: error: cannot cast 'const std::variant<io_outcome::success, io_outcome::not_found>' to its private base class 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:1255:35: note: in instantiation of function template specialization 'std::__detail::__variant::__get<0, const std::variant<io_outcome::success, io_outcome::not_found> >' requested here
      return __detail::__variant::__get<_Np>(std::move(__v));
                                  ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:676:9: note: in instantiation of function template specialization 'std::get<0, io_outcome::success, io_outcome::not_found>' requested here
                        std::get<__indices>(
                             ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:682:29: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 0> >::__visit_invoke' requested here
      { return _Array_type{&__visit_invoke}; }
                            ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:659:53: note: in instantiation of member function 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long, 0> >::_S_apply' requested here
              std::index_sequence<__indices..., __index>>::_S_apply();
                                                           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:647:5: note: in instantiation of function template specialization 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&), 2>, std::tuple<std::variant<io_outcome::success, io_outcome::not_found> &&>, std::integer_sequence<unsigned long> >::_S_apply_single_alt<0, std::__detail::__variant::_Multi_array<int (*)(scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)> &&, std::variant<io_outcome::success, io_outcome::not_found> &&)> >' requested here
          (_S_apply_single_alt<__var_indices>(
           ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:637:2: note: (skipping 4 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        _S_apply_all_alts(
        ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/std.hpp:16:58: note: in instantiation of function template specialization 'std::visit<scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, std::variant<io_outcome::success, io_outcome::not_found> >' requested here
SCELTA_DEFINE_HOMOGENIZER_VARIANT(::std::variant, ::std::visit)
                                                         ^
/home/jackie/code/scelta/include/./scelta/./support/./variant/../../utils/homogenizer.hpp:36:13: note: while substituting deduced template arguments into function template 'operator()' [with Ts = <scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, std::variant<io_outcome::success, io_outcome::not_found>>]
            homogenizer_helper_t<std::decay_t<Variant>>{}(
            ^
/home/jackie/code/scelta/include/./scelta/./recursive/../visitation/./visit.hpp:21:13: note: while substituting deduced template arguments into function template 'visit_homogenizer' [with Visitor = scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            impl::visit_homogenizer(
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:38:13: note: while substituting deduced template arguments into function template 'visit' [with Visitor = scelta::overloader<(lambda at /home/jackie/code/scelta/example/error_handling.cpp:54:9), (lambda at /home/jackie/code/scelta/example/error_handling.cpp:55:9)>, Variant = std::variant<io_outcome::success, io_outcome::not_found>, Variants = <>]
            DEFINE_BOUND_VISITOR_CALL(Visitor&&,      &&)
            ^
/home/jackie/code/scelta/include/./scelta/./visitation/match.hpp:30:13: note: expanded from macro 'DEFINE_BOUND_VISITOR_CALL'
            visit(std::declval<m_visitor_type>(), FWD(variants)...)         \
            ^
/home/jackie/code/scelta/example/error_handling.cpp:56:9: note: while substituting deduced template arguments into function template 'operator()' [with Variants = <std::variant<io_outcome::success, io_outcome::not_found>>]
        (read_file(argv[1]));
        ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:235:74: error: '_M_u' is a private member of 'std::__detail::__variant::_Variant_storage<false, io_outcome::success, io_outcome::not_found>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                                                         ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:906:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.1/../../../../include/c++/7.0.1/variant:356:34: note: member is declared here
      _Variadic_union<_Types...> _M_u;
                                 ^
8 errors generated.
make[2]: *** [example/CMakeFiles/example_error_handling.dir/error_handling.cpp.o] Error 1
make[1]: *** [example/CMakeFiles/example_error_handling.dir/all] Error 2
make: *** [all] Error 2

I got a similar error today with the libstdc++ version of std::variant when compiling with clang-SVN (gcc 7.1 works fine). I found the following LLVM bug report for it. FWIW, my example is too big for bug isolation, but it went away using Boost.Variant.