Initialize constexpr with extern constant

Question

I would like to initialize a constexpr variable with another variable, made visible by extern:

test.cpp

#include "test.h"
int const a[] = {1};
int const d = 4;

test.h

extern int const a[];
extern int const d;

main.cpp

#include "test.h"

int main(void) {
    constexpr const int * b = a;
    constexpr const int e = d;
}

(Only) The second line in main gives the following error:

error: the value of 'd' is not usable in a constant expression and

test.h:3:18: note: 'd' was not initialized with a constant expression

Could somebody help me to understand, why only one of the two examples fails?