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#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;
}

prog.cpp: In function 'int main()':
prog.cpp:13:30: error: the value of 'PI1__' is not usable in a constant expression
     constexpr double PI3__ = PI1__;  // error
                              ^
prog.cpp:11:18: note: 'PI1__' was not declared 'constexpr'
     const double PI1__ = 3;
                  ^

Standard output is empty