std::variant with const arguments in C++

  c++, variant

There is a nice unanswered question about having union with const members: Can you write a copy constructor for a union with const members?

One of suggestions there is to use std::variant instead. Indeed, const types must be supported P0086 – Variant design review. The relevant paragraph says:

variant<int, const int> A variant can handle const types: they
can only be set through variant construction and emplace().

So I assume variant copy construction must support them as well.

But an attempt to use this option:

#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;
}

fails in GCC with a very long error (quote only its beginning here):

n 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

Is there something wrong with my code, or GCC must accept it?

Source: Windows Questions C++

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