Pattern Matching C++ paper example not working - c++

Question

I've come across the pattern matching proposal in C++, and I tried to compile Example 4.6 about expression trees.
Here's the extracted code:
#include <memory>
#include <variant>
struct Expr;
struct Neg {
std::shared_ptr<Expr> expr;
};
struct Add {
std::shared_ptr<Expr> lhs, rhs;
};
struct Mul {
std::shared_ptr<Expr> lhs, rhs;
};
struct Expr : std::variant<int, Neg, Add, Mul> {
using variant::variant;
};
namespace std {
template <>
struct variant_size<Expr> : variant_size<Expr::variant> {};
template <std::size_t I>
struct variant_alternative<I, Expr> : variant_alternative<I, Expr::variant> {};
}
int eval(const Expr& expr) {
struct visitor {
int operator()(int i) const {return i;}
int operator()(const Neg& n) const {return -eval(*n.expr);}
int operator()(const Add& a) const {return eval(*a.lhs) + eval(*a.rhs);}
int operator()(const Mul& m) const {// Optimize multiplication by 0.
if (int* i = std::get_if<int>(m.lhs.get()); i && *i == 0) {
return 0;
}
if (int* i = std::get_if<int>(m.rhs.get()); i && *i == 0) {
return 0;
}
return eval(*m.lhs) * eval(*m.rhs);}
};
return std::visit(visitor{}, expr);
}
The code above gives 4 compilation errors, with:
clang++ -std=c++17 main.cpp
In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:778:6: error: implicit instantiation of undefined template 'std::__detail::__variant::_Extra_visit_slot_needed<int, const Expr &>::_Variant_never_valueless<Expr>'
&& !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:809:44: note: in instantiation of static data member 'std::__detail::__variant::_Extra_visit_slot_needed<int, const Expr &>::value' requested here
_Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:53: note: in instantiation of static data member 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>::__do_cookie' requested here
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
static constexpr _Array_type _S_vtable
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
constexpr auto& __vtable = __detail::__variant::__gen_vtable<
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
return __do_visit(std::forward<_Visitor>(__visitor),
^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
return std::visit(visitor{}, expr);
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:769:34: note: template is declared here
template <typename> struct _Variant_never_valueless;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:809:2: error: constexpr variable '__do_cookie' must be initialized by a constant expression
_Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:53: note: in instantiation of static data member 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>::__do_cookie' requested here
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
static constexpr _Array_type _S_vtable
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
constexpr auto& __vtable = __detail::__variant::__gen_vtable<
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
return __do_visit(std::forward<_Visitor>(__visitor),
^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
return std::visit(visitor{}, expr);
^
In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:36: error: fields must have a constant size: 'variable length array in structure' extension will never be supported
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
static constexpr _Array_type _S_vtable
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
constexpr auto& __vtable = __detail::__variant::__gen_vtable<
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
return __do_visit(std::forward<_Visitor>(__visitor),
^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
return std::visit(visitor{}, expr);
^
In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: error: constexpr variable '_S_vtable' must be initialized by a constant expression
static constexpr _Array_type _S_vtable
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1640:45: note: in instantiation of static data member 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>::_S_vtable' requested here
_Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
return __do_visit(std::forward<_Visitor>(__visitor),
^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
return std::visit(visitor{}, expr);
^
4 errors generated.
How to make the code above to compile?
Basically, the question is how to have recursive types with std::variant and std::shared_ptr for incomplete types.
The command clang++ --version gives:
clang version 9.0.0-2 (tags/RELEASE_900/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin