Conditionally disabling a copy constructor

Question

Suppose I\'m writing a class template C that holds a T value, so C can be copyable only if T is copyabl

Answer 1

template 
class variant {
    struct moo {};
public:
  variant(const variant& ) = default;
  variant(std::conditional_t::value,
                             const variant&, moo>,
          moo=moo());
  variant() {};
};

This makes a non-eligible template instance have two copy constructors, which makes it not copy constructible.