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#include <iostream>
#include <type_traits>
 
template<typename T>
class ptr {
	T* m_ptr;
    public:
		ptr(T* a_ptr) : m_ptr(a_ptr) {}
 
		template<typename U = T>
		typename
			std::enable_if<!std::is_same<U, void>::value, U&>::type
		operator * () const { return *m_ptr; };
};
 
int main()
{
	int i = 12345;
	ptr<int> ptr_i(&i);
	std::cout << *ptr_i; // OK
 
	ptr<void> ptr_i2(&i);
	std::cout << *ptr_i2; // ERROR
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:23:15: error: no match for ‘operator*’ (operand type is ‘ptr<void>’)
  std::cout << *ptr_i2; // ERROR
               ^~~~~~~
prog.cpp:13:3: note: candidate: ‘template<class U> typename std::enable_if<(! std::is_same<U, void>::value), U&>::type ptr<T>::operator*() const [with U = U; T = void]’
   operator * () const { return *m_ptr; };
   ^~~~~~~~
prog.cpp:13:3: note:   template argument deduction/substitution failed:
prog.cpp: In substitution of ‘template<class U> typename std::enable_if<(! std::is_same<U, void>::value), U&>::type ptr<void>::operator*<U>() const [with U = void]’:
prog.cpp:23:16:   required from here
prog.cpp:13:3: error: forming reference to void

Standard output is empty