#include <iostream>
using namespace std;

int main() 
{
   const int PI1 = 3;
    constexpr int PI2 = 3;
    constexpr int PI3 = PI1;  // works
    static_assert(PI1 == 3, "");  // works

    const double PI1__ = 3;
    constexpr double PI2__ = 3;
    constexpr double PI3__ = PI1__;  // error
	return 0;
}