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#include <iostream>
#include <type_traits>
 
template <typename T>
struct vec;
 
template <typename T, bool B>
struct vec_impl;
 
template <typename T>
struct vec_impl<T, true> {
	static void add(const vec<T>& self, const T& a) {
		T n(a);
	}
};
 
template <typename T>
class vec {
	friend struct vec_impl<T, true>;
 
public:
	virtual void add(const T& a) {
		vec_impl<T, std::is_copy_constructible<T>::value>::add(*this, a);
	}
 
	virtual void add(T&& a) {
 
	}
};
 
struct mov {
	mov() = default;
	mov(const mov&) = delete;
	mov(mov&&) = default;
};
 
int main() {
	mov m;
 
	vec<mov> v;
 
	v.add(std::move(m));
 
	return 0;
}

prog.cpp: In instantiation of ‘void vec<T>::add(const T&) [with T = mov]’:
prog.cpp:45:1:   required from here
prog.cpp:23:66: error: incomplete type ‘vec_impl<mov, false>’ used in nested name specifier
   vec_impl<T, std::is_copy_constructible<T>::value>::add(*this, a);
                                                                  ^

Standard output is empty