constexpr vs const vs constexpr const

Question

const-vs-constexpr-on-variables

What the guy says about constexpr is right if double is used (or float of course). However, if you change t

Answer 1

The compiler does not allow an implicit narrowing or non-integral promotion during the initialisation of a constexpr variable.

This will work:

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__ = double(PI1);  // works with explicit cast
    static_assert(PI2__ == 3, "");  // works now. PI1__ isn't constexpr
    return 0;
}