[Bug libstdc++/79513] New: std::visit doesn't handle references

["benni.buch at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79513

            Bug ID: 79513
           Summary: std::visit doesn't handle references
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: benni.buch at gmail dot com
  Target Milestone: ---

#include <variant>

int main(){
        std::variant<int> v(5);
        std::visit([](int&){}, v);
        std::visit([](int&&){}, std::move(v));
}


Should be a valid program, but libstdc++ discards in both cases the references:

In file included from main.cpp:1:0:
/usr/local/include/c++/7.0.1/variant: In instantiation of 'constexpr
decltype(auto) std::visit(_Visitor&&, _Variants&& ...) [with _Visitor =
main()::<lambda(int&&)>; _Variants = {std::variant<int>}]':
main.cpp:6:38:   required from here
/usr/local/include/c++/7.0.1/variant:1266:44: error: no match for call to
'(main()::<lambda(int&&)>) (std::variant_alternative_t<0, std::variant<int>
>&)'
  decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/c++/7.0.1/variant:1266:44: note: candidate: void (*)(int&&)
<conversion>
/usr/local/include/c++/7.0.1/variant:1266:44: note:   conversion of argument 2
would be ill-formed:
/usr/local/include/c++/7.0.1/variant:1266:44: error: cannot bind rvalue
reference of type 'int&&' to lvalue of type 'std::variant_alternative_t<0,
std::variant<int> > {aka int}'
main.cpp:6:21: note: candidate: main()::<lambda(int&&)> <near match>
  std::visit([](int&&){}, std::move(v));
                     ^
main.cpp:6:21: note:   conversion of argument 1 would be ill-formed:
In file included from main.cpp:1:0:
/usr/local/include/c++/7.0.1/variant:1266:44: error: cannot bind rvalue
reference of type 'int&&' to lvalue of type 'std::variant_alternative_t<0,
std::variant<int> > {aka int}'
  decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/c++/7.0.1/variant:1266:44: error: no match for call to
'(main()::<lambda(int&&)>) (std::variant_alternative_t<0, std::variant<int>
>&)'
/usr/local/include/c++/7.0.1/variant:1266:44: note: candidate: void (*)(int&&)
<conversion>
/usr/local/include/c++/7.0.1/variant:1266:44: note:   conversion of argument 2
would be ill-formed:
/usr/local/include/c++/7.0.1/variant:1266:44: error: cannot bind rvalue
reference of type 'int&&' to lvalue of type 'std::variant_alternative_t<0,
std::variant<int> > {aka int}'
main.cpp:6:21: note: candidate: main()::<lambda(int&&)> <near match>
  std::visit([](int&&){}, std::move(v));
                     ^
main.cpp:6:21: note:   conversion of argument 1 would be ill-formed:
In file included from main.cpp:1:0:
/usr/local/include/c++/7.0.1/variant:1266:44: error: cannot bind rvalue
reference of type 'int&&' to lvalue of type 'std::variant_alternative_t<0,
std::variant<int> > {aka int}'
  decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~