compile error on requires to_view

[zhuoqiang]

I want to check if rfl::to_view() can be applied to a variable of type T, but the following code fails to compile:

template<class T>
void FailedToCompile(T const& t) {
    if constexpr (requires { rfl::to_view(t); }) {
    }
}

int main() {
    std::string s = "hello";
    FailedToCompile(s);
}

As a workaround, I have to use std::is_aggregate_v<T> for the check instead.

Would it be possible to add a concept constraint requiring rfl::to_view to only be applicable when T satisfies std::is_aggregate_v<T>? That way, the if constexpr (requires { rfl::to_view(t); }) condition would work as expected.

[liuzicheng1987]

Hi @zhuoqiang , what error message do you get when you do this?

[zhuoqiang]

@liuzicheng1987

In file included from /rfl/internal/get_ith_field_from_fake_object.hpp:13,
                 from /rfl/internal/get_field_names.hpp:12,
                 from /rfl/field_names_t.hpp:7,
                 from /rfl/internal/to_ptr_named_tuple.hpp:9,
                 from /rfl/to_view.hpp:8,
                 from main.cpp:1:
/rfl/internal/num_fields.hpp: In instantiation of 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_max_args_in_agg() [with long unsigned int n = 33; T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]':
/rfl/internal/num_fields.hpp:107:42:   required from 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_fields() [with T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]'
  107 |       return count_max_args_in_agg<n + 1>();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/num_fields.hpp:208:70:   required from 'constexpr const std::size_t rfl::internal::num_fields<std::__cxx11::basic_string<char> >'
  208 | constexpr std::size_t num_fields = CountFieldsHelper<T>::count_fields();
      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/bind_to_tuple.hpp:551:28:   required from 'auto rfl::internal::tuple_view(T&) [with T = std::__cxx11::basic_string<char>]'
  551 |   return tuple_view_helper<num_fields<T>>::tuple_view(t);
      |                            ^~~~~~~~~~~~
/rfl/internal/bind_to_tuple.hpp:556:25:   required from 'auto rfl::internal::bind_to_tuple(T&, const F&) [with T = std::__cxx11::basic_string<char>; F = to_ptr_tuple<std::__cxx11::basic_string<char> >(std::__cxx11::basic_string<char>&)::<lambda(auto:362*)>]'
  556 |   auto view = tuple_view(_t);
      |               ~~~~~~~~~~^~~~
/rfl/internal/to_ptr_tuple.hpp:18:25:   required from 'auto rfl::internal::to_ptr_tuple(T&) [with T = std::__cxx11::basic_string<char>]'
   18 |     return bind_to_tuple(_t, [](auto* _ptr) { return _ptr; });
      |            ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/rfl/internal/ptr_tuple_t.hpp:13:42:   required by substitution of 'template<class T> using rfl::internal::ptr_tuple_t = decltype (rfl::internal::to_ptr_tuple(declval<T&>())) [with T = std::__cxx11::basic_string<char>]'
   13 | using ptr_tuple_t = decltype(to_ptr_tuple(std::declval<T&>()));
      |                              ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
/rfl/internal/has_fields.hpp:61:11:   required from 'constexpr bool rfl::internal::has_fields() [with T = std::__cxx11::basic_string<char>]'
   61 |     using TupleType = ptr_tuple_t<T>;
      |           ^~~~~~~~~
/rfl/internal/to_ptr_named_tuple.hpp:61:51:   required from 'auto rfl::internal::to_ptr_named_tuple(T&&) [with T = const std::__cxx11::basic_string<char>&]'
   61 |   if constexpr (has_fields<std::remove_cvref_t<T>>()) {
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/to_view.hpp:14:38:   required from 'auto rfl::to_view(T&) [with T = const std::__cxx11::basic_string<char>]'
   14 |   return internal::to_ptr_named_tuple(_t);
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
main.cpp:8:40:   required from 'void FailedToCompile(const T&) [with T = std::__cxx11::basic_string<char>]'
    8 |   if constexpr (requires { rfl::to_view(t); }) {
      |                            ~~~~~~~~~~~~^~~
main.cpp:14:18:   required from here
   14 |   FailedToCompile(s);
      |   ~~~~~~~~~~~~~~~^~~
/rfl/internal/num_fields.hpp:184:63:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<1>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<2>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<3>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<4>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<5>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<6>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<7>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<8>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<9>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<10>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<11>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<12>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<13>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<14>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<15>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<16>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<17>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<18>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<19>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<20>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<21>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<22>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<23>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<24>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<25>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<26>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<27>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<28>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<29>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<30>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<31>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<32>()'
/rfl/internal/num_fields.hpp:103:21: error: static assertion failed
  103 |     static_assert(n <= static_cast<std::size_t>(sizeof(T)));
      |                   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/rfl/internal/num_fields.hpp:103:21: note: the comparison reduces to '(33 <= 32)'
/rfl/internal/num_fields.hpp: In instantiation of 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_max_args_in_agg() [with long unsigned int n = 34; T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]':
/rfl/internal/num_fields.hpp:107:42:   required from 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_fields() [with T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]'
  107 |       return count_max_args_in_agg<n + 1>();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/num_fields.hpp:208:70:   required from 'constexpr const std::size_t rfl::internal::num_fields<std::__cxx11::basic_string<char> >'
  208 | constexpr std::size_t num_fields = CountFieldsHelper<T>::count_fields();
      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/bind_to_tuple.hpp:551:28:   required from 'auto rfl::internal::tuple_view(T&) [with T = std::__cxx11::basic_string<char>]'
  551 |   return tuple_view_helper<num_fields<T>>::tuple_view(t);
      |                            ^~~~~~~~~~~~
/rfl/internal/bind_to_tuple.hpp:556:25:   required from 'auto rfl::internal::bind_to_tuple(T&, const F&) [with T = std::__cxx11::basic_string<char>; F = to_ptr_tuple<std::__cxx11::basic_string<char> >(std::__cxx11::basic_string<char>&)::<lambda(auto:362*)>]'
  556 |   auto view = tuple_view(_t);
      |               ~~~~~~~~~~^~~~
/rfl/internal/to_ptr_tuple.hpp:18:25:   required from 'auto rfl::internal::to_ptr_tuple(T&) [with T = std::__cxx11::basic_string<char>]'
   18 |     return bind_to_tuple(_t, [](auto* _ptr) { return _ptr; });
      |            ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/rfl/internal/ptr_tuple_t.hpp:13:42:   required by substitution of 'template<class T> using rfl::internal::ptr_tuple_t = decltype (rfl::internal::to_ptr_tuple(declval<T&>())) [with T = std::__cxx11::basic_string<char>]'
   13 | using ptr_tuple_t = decltype(to_ptr_tuple(std::declval<T&>()));
      |                              ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
/rfl/internal/has_fields.hpp:61:11:   required from 'constexpr bool rfl::internal::has_fields() [with T = std::__cxx11::basic_string<char>]'
   61 |     using TupleType = ptr_tuple_t<T>;
      |           ^~~~~~~~~
/rfl/internal/to_ptr_named_tuple.hpp:61:51:   required from 'auto rfl::internal::to_ptr_named_tuple(T&&) [with T = const std::__cxx11::basic_string<char>&]'
   61 |   if constexpr (has_fields<std::remove_cvref_t<T>>()) {
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/to_view.hpp:14:38:   required from 'auto rfl::to_view(T&) [with T = const std::__cxx11::basic_string<char>]'
   14 |   return internal::to_ptr_named_tuple(_t);
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
main.cpp:8:40:   required from 'void FailedToCompile(const T&) [with T = std::__cxx11::basic_string<char>]'
    8 |   if constexpr (requires { rfl::to_view(t); }) {
      |                            ~~~~~~~~~~~~^~~
main.cpp:14:18:   required from here
   14 |   FailedToCompile(s);
      |   ~~~~~~~~~~~~~~~^~~
/rfl/internal/num_fields.hpp:184:63:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<1>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<2>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<3>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<4>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<5>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<6>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<7>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<8>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<9>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<10>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<11>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<12>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<13>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<14>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<15>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<16>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<17>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<18>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<19>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<20>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<21>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<22>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<23>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<24>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<25>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<26>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<27>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<28>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<29>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<30>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<31>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<32>()'
/rfl/internal/num_fields.hpp:107:42:   in 'constexpr' expansion of 'rfl::internal::CountFieldsHelper<std::__cxx11::basic_string<char> >::count_max_args_in_agg<33>()'
/rfl/internal/num_fields.hpp:103:21: error: static assertion failed
  103 |     static_assert(n <= static_cast<std::size_t>(sizeof(T)));
      |                   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/rfl/internal/num_fields.hpp:103:21: note: the comparison reduces to '(34 <= 32)'
/rfl/internal/num_fields.hpp: In instantiation of 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_max_args_in_agg() [with long unsigned int n = 35; T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]':
/rfl/internal/num_fields.hpp:107:42:   required from 'static consteval std::size_t rfl::internal::CountFieldsHelper<T>::count_fields() [with T = std::__cxx11::basic_string<char>; std::size_t = long unsigned int]'
  107 |       return count_max_args_in_agg<n + 1>();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/num_fields.hpp:208:70:   required from 'constexpr const std::size_t rfl::internal::num_fields<std::__cxx11::basic_string<char> >'
  208 | constexpr std::size_t num_fields = CountFieldsHelper<T>::count_fields();
      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/internal/bind_to_tuple.hpp:551:28:   required from 'auto rfl::internal::tuple_view(T&) [with T = std::__cxx11::basic_string<char>]'
  551 |   return tuple_view_helper<num_fields<T>>::tuple_view(t);
      |                            ^~~~~~~~~~~~
/rfl/internal/bind_to_tuple.hpp:556:25:   required from 'auto rfl::internal::bind_to_tuple(T&, const F&) [with T = std::__cxx11::basic_string<char>; F = to_ptr_tuple<std::__cxx11::basic_string<char> >(std::__cxx11::basic_string<char>&)::<lambda(auto:362*)>]'
  556 |   auto view = tuple_view(_t);
      |               ~~~~~~~~~~^~~~
/rfl/internal/to_ptr_tuple.hpp:18:25:   required from 'auto rfl::internal::to_ptr_tuple(T&) [with T = std::__cxx11::basic_string<char>]'
   18 |     return bind_to_tuple(_t, [](auto* _ptr) { return _ptr; });
      |            ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/rfl/internal/ptr_tuple_t.hpp:13:42:   required by substitution of 'template<class T> using rfl::internal::ptr_tuple_t = decltype (rfl::internal::to_ptr_tuple(declval<T&>())) [with T = std::__cxx11::basic_string<char>]'
   13 | using ptr_tuple_t = decltype(to_ptr_tuple(std::declval<T&>()));
      |                              ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
/rfl/internal/has_fields.hpp:61:11:   required from 'constexpr bool rfl::internal::has_fields() [with T = std::__cxx11::basic_string<char>]'
   61 |     using TupleType = ptr_tuple_t<T>;
      |           ^~~~~~~~~
/rfl/internal/to_ptr_named_tuple.hpp:61:51:   required from 'auto rfl::internal::to_ptr_named_tuple(T&&) [with T = const std::__cxx11::basic_string<char>&]'
   61 |   if constexpr (has_fields<std::remove_cvref_t<T>>()) {
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/rfl/to_view.hpp:14:38:   required from 'auto rfl::to_view(T&) [with T = const std::__cxx11::basic_string<char>]'
   14 |   return internal::to_ptr_named_tuple(_t);
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
main.cpp:8:40:   required from 'void FailedToCompile(const T&) [with T = std::__cxx11::basic_string<char>]'
    8 |   if constexpr (requires { rfl::to_view(t); }) {
      |                            ~~~~~~~~~~~~^~~
main.cpp:14:18:   required from here
   14 |   FailedToCompile(s);
      |   ~~~~~~~~~~~~~~~^~~

[liuzicheng1987]

Interesting...this apparently happens when you try to insert a string.

I think this could be resolved with a well-placed constexpr if.

I will take a look.

[zhuoqiang]

no need to do actual actions, the following constexpr check is enough to reproduce the compile error:

if constexpr (requires { rfl::to_view(t); }) {
}