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#include<iostream>
 
class Point
{
public:
	int X, Y;
	mutable int Z;
 
	constexpr Point(int x, int y) :X (x), Y(y), Z(0)
	{ }
 
	constexpr int GetX() const
	{
		// Z++;
		return X+Z;
	}
 
	int GetY() const
	{
		Z++;
		return Y;
	}
 
	void FoolConst() const
	{
		Z++;
	}
};
 
constexpr int foo()
{
	return 0;
}
 
template<int S>
void foo()
{
	std::cout<<S << std::endl;
}
 
int main()
{	
	constexpr Point pt(10, 20);
 
	pt.FoolConst();
 
	foo<pt.GetX()>();
 
	std::cout << pt.GetX();
}

prog.cpp: In function 'int main()':
prog.cpp:47:17: error: no matching function for call to 'foo()'
  foo<pt.GetX()>();
                 ^
prog.cpp:36:6: note: candidate: template<int S> void foo()
 void foo()
      ^
prog.cpp:36:6: note:   template argument deduction/substitution failed:
prog.cpp:47:13:   in constexpr expansion of 'pt.Point::GetX()'
prog.cpp:47:17: error: mutable 'Point::Z' is not usable in a constant expression
  foo<pt.GetX()>();
                 ^
prog.cpp:47:17: note: in template argument for type 'int' 

Standard output is empty