[lucic71]

Instantiated templates can be generated as structs with zero fields, aka unit struct.

We need unit structs to be copyable, clonable, defaultable and byterepr because they can be included in other structs that already satisfy those traits. For example:

// This will be converted as a unit struct, it has no fields
class Probe {
public:
  Probe &operator++();
};

// Wrapper contains a unit struct, the unit struct must be clonable, copyable, etc,
// so that Wrapper can be clonable, copyable, etc
template <class T>
struct Wrapper {
  T base_;
  int tag;
};

int main() {
  // Instantiating Wrapper with Probe
  Wrapper<Probe> a{};
}

I also implemented ByteRepr as a proc-macro. Currently it only supports unit structs.