Static_cast and templated conversion function - c++

Question

I have a class that wraps some type and attaches a Dimension. It should be convertible to the underlying type, but only if Dim=0. The conversion operator should not be callable in other cases (so a static_assert in the function would not work for me).
The following code works, if the enable_if-construction is deleted, but not in this form.
template<class T, int Dim>
class Unit {
public:
explicit Unit(T const& value): _value(value) {}
template<int D = Dim, typename = typename std::enable_if<D == 0>::type>
operator T() { return _value; }
private:
T _value;
};
auto main() -> int
{
auto a = double{0};
auto u = Unit<double, 0>{a};
auto i = static_cast<int>(u);
return i;
}
What is the reason for this and is there a work around to allow the cast, but also restrict the conversion?

Answer 1

As I understand, you want:
template <class T, int Dim>
class Unit {
public:
explicit Unit(T const& value): _value(value) {}
template <typename U, int D = Dim,
std::enable_if_t<D == 0 && std::is_convertible_v<T, U>, int> = 0>
operator U() { return _value; }
private:
T _value;
};
Demo
And in C++20, look nicer
template<class T, int Dim>
class Unit {
public:
explicit Unit(T const& value): _value(value) {}
template <typename U>
requires(Dim == 0 && std::is_convertible_v<T, U>)
operator U() const { return _value; }
private:
T _value;
};
Demo