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#include <iostream>
 
double add_one (double x)
{
	return x+1;
}
 
template <typename Type, Type Instance>
struct test_type
{
	double operator() (double x)
	{
		return Instance (x);
	}
};
 
int main ()
{
	test_type<decltype(add_one), add_one> test;
 
	std::cout << test(10.0) << "\n";
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:19:38: error: ‘double(double)’ is not a valid type for a template non-type parameter
  test_type<decltype(add_one), add_one> test;
                                      ^
prog.cpp:19:44: error: invalid type in declaration before ‘;’ token
  test_type<decltype(add_one), add_one> test;
                                            ^
prog.cpp:21:24: error: ‘test’ cannot be used as a function
  std::cout << test(10.0) << "\n";
                        ^

Standard output is empty