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 the var type from double to an integer type like int, char, etc, everything works. Why does that happen?

http://ideone.com/DAWABE

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

    const double PI1__ = 3.0;
    constexpr double PI2__ = 3.0;
    constexpr double PI3__ = PI1__;  // error
    static_assert(PI1__ == 3.0, "");  // error
    return 0;
}

Update: the following line was a mistake, I meant PI3__ = PI1__

constexpr double PI3__ = PI1;  // I meant PI1__

Questions:

1. Why const int = 3 is compile time constant but const double = 3.0 is not?

2. Is there any reason why I should use constexpr const int val; over constexpr int val? They both seem to do exactly the same.

Answer 1

From the comments it seems like OP is asking for Standard quote which defines const int as a compile-time constant, but const double as not.

The corresponding details are found in 5.19, Constant Expressions. In particular:

...an lvalue-to-rvalue conversion (4.1) unless it is applied to a non-volatile glvalue of integral or enumeration type that refers to a non-volatile const object with a preceding initialization, initialized with a constant expression...

int is an integral type, while double is not.