[Bug libstdc++/95904] New: Improve the diagnostic for conflicting return types in std::visit

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95904

            Bug ID: 95904
           Summary: Improve the diagnostic for conflicting return types in
                    std::visit
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

std::variant<int, long> x;
std::visit([] (auto i) { return i; }, x);

produces a less than ideal error with libstdc++:

In file included from <source>:1:

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant: In
instantiation of 'static constexpr auto
std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<_Result_type
(*)(_Visitor, _Variants ...)>, std::integer_sequence<long unsigned int,
__indices ...> >::_S_apply() [with _Result_type =
std::__detail::__variant::__deduce_visit_result<int>; _Visitor =
main()::<lambda(auto:22)>&&; _Variants = {std::variant<int, long int>&}; long
unsigned int ...__indices = {1}]':

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:962:56:  
required from 'static constexpr void
std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<_Result_type
(*)(_Visitor, _Variants ...), __dimensions ...>, std::integer_sequence<long
unsigned int, __indices ...> >::_S_apply_single_alt(_Tp&, _Tp*) [with bool
__do_cookie = false; long unsigned int __index = 1; _Tp =
std::__detail::__variant::_Multi_array<std::__detail::__variant::__deduce_visit_result<int>
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)>; _Result_type =
std::__detail::__variant::__deduce_visit_result<int>; _Visitor =
main()::<lambda(auto:22)>&&; long unsigned int ...__dimensions = {2}; _Variants
= {std::variant<int, long int>&}; long unsigned int ...__indices = {}]'

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:941:48:  
required from 'constexpr const _Array_type
std::__detail::__variant::__gen_vtable<std::__detail::__variant::__deduce_visit_result<int>,
main()::<lambda(auto:22)>&&, std::variant<int, long int>&>::_S_vtable'

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:1685:45:  
required from 'constexpr decltype(auto) std::__do_visit(_Visitor&&, _Variants&&
...) [with _Result_type = std::__detail::__variant::__deduce_visit_result<int>;
_Visitor = main()::<lambda(auto:22)>; _Variants = {std::variant<int, long
int>&}]'

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:1704:35:  
required from 'constexpr decltype(auto) std::visit(_Visitor&&, _Variants&& ...)
[with _Visitor = main()::<lambda(auto:22)>; _Variants = {std::variant<int, long
int>&}]'

<source>:5:43:   required from here

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:1028:67:  
in 'constexpr' expansion of
'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::__detail::__variant::__deduce_visit_result<int>
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&), 2>,
std::integer_sequence<long unsigned int> >::_S_apply()'

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:926:19:  
in 'constexpr' expansion of
'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::__detail::__variant::__deduce_visit_result<int>
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&), 2>,
std::integer_sequence<long unsigned int> >::_S_apply_all_alts<0, 1>(__vtable,
(std::make_index_sequence<2>(), std::make_index_sequence<2>()))'

/opt/compiler-explorer/gcc-trunk-20200625/include/c++/11.0.0/variant:1017:43:
error: invalid conversion from 'std::__success_type<long int>::type
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)' {aka 'long int
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)'} to
'std::__detail::__variant::_Multi_array<std::__detail::__variant::__deduce_visit_result<int>
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long
int>&)>::__untag_result<std::__detail::__variant::__deduce_visit_result<int>
(*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)>::element_type'
{aka 'int (*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)'}
[-fpermissive]

 1017 |       { return _Array_type{&__visit_invoke}; }

      |                                           ^

      |                                           |

      |                                           std::__success_type<long
int>::type (*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&) {aka
long int (*)(main()::<lambda(auto:22)>&&, std::variant<int, long int>&)}

Not only is the error about type mismatch in some function pointer conversion
deep in the guts of the library, but it even offers a "helpful" option to
suppress it with -fpermissive...

Both MSVC and libc++ handle this case with a nice static_assert.