https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102912
Bug ID: 102912
Summary: Not full support of const arguments in std::variant
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: fchelnokov at gmail dot com
Target Milestone: ---
This program is valid:
```
#include <string>
#include <variant>
using S = std::variant<const int, const std::string>;
int main() {
S s(1);
S u = s;
S v("abc");
S w = v;
}
```
but its compilation in GCC results in a long error:
```
In file included from <source>:2:
/opt/compiler-explorer/gcc-trunk-20211024/include/c++/12.0.0/variant: In
instantiation of 'constexpr std::__detail::__variant::_Variadic_union<_First,
_Rest ...>::_Variadic_union(std::in_place_index_t<_Np>, _Args&& ...) [with long
unsigned int _Np = 1; _Args = {const int&}; _First = const
std::__cxx11::basic_string<char>; _Rest = {}]':
/opt/compiler-explorer/gcc-trunk-20211024/include/c++/12.0.0/variant:409:4:
required from 'constexpr std::__detail::__variant::_Variadic_union<_First,
_Rest ...>::_Variadic_union(std::in_place_index_t<_Np>, _Args&& ...) [with long
unsigned int _Np = 2; _Args = {const int&}; _First = const int; _Rest = {const
std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>
>}]'
/opt/compiler-explorer/gcc-trunk-20211024/include/c++/12.0.0/bits/stl_construct.h:119:7:
required from 'constexpr void std::_Construct(_Tp*, _Args&& ...) [with _Tp =
std::__detail::__variant::_Variadic_union<const int, const
std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >
>; _Args = {const std::in_place_index_t<2>&, const int&}]'
...
Demo: https://gcc.godbolt.org/z/ocajj3aao
Related discussion: https://stackoverflow.com/q/69696170/7325599
```
