Recursive variant definition within template class scope gives incomplete type error, why?

I tried to adapt this article to my context where the recursive variant should be defined within the scope of a template class.

Simplifying everything to the extreme, this gives me the following code snippet:

#include <variant>

struct LiteralExpr
{
    int value;
};

template <typename T>
struct BaseSpace
{
    struct AddExpr;
    struct AddExprBox
    {
        const AddExpr &_impl;

    public:
        AddExprBox(const AddExpr &obj) {}
    };
    using Expr = std::variant<AddExprBox, LiteralExpr>;
    struct AddExpr
    {
        std::variant<AddExprBox, LiteralExpr> lhs;
    };
};

auto main() -> int
{
    auto expr = BaseSpace<int>::Expr(LiteralExpr{2});
}

Sadly, this fails to compile on clang with the following error:

main.cpp:22:47: error: field has incomplete type 'std::variant<AddExprBox, LiteralExpr>' std::variant<AddExprBox, LiteralExpr> lhs;

If:

• I remove the copy constructor within AddExprBox, it compiles;
• I make Basespace a "non-template" class, it compiles;
• I explicitly instantiate `BaseSpace, it compiles;

Any idea of why this behavior occurs? And how to fix it?