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#include <iostream>
using namespace std;
 
template<typename T>
class Rect
{
	using val_t = T;
public:
	Rect(const val_t x0, const val_t y0, const val_t x, const val_t y) 
		: p0{ x0, y0 }, p{ x, y } {}
 
	val_t left() const { return p0.x; }
	val_t bottom() const { return p0.y; }
	val_t right() const { return p.x; }
	val_t top() const { return p.y; }
 
private:
	struct point
	{
		val_t x = 0;
		val_t y = 0;
	} p0, p;
};
 
int main() {
	Rect<int> r(0, 0, 1, 1);
 
	decltype(r)::val_t x = r.left();
 
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:28:15: error: ‘using val_t = int’ is private within this context
  decltype(r)::val_t x = r.left();
               ^~~~~
prog.cpp:7:17: note: declared private here
  using val_t = T;
                 ^

Standard output is empty