why it is not legal to access gloabl non constant vairable in constexpr non member function

int d = 1;
constexpr void add()
{
    d++;
}

int main()
{

}

GCC 7.1 will report errors below. The error message is very clear. The problem is that I don't see any explicit explanations in constexpr to describe it's illegal.

Can someone explain which rules defined in spec this case violate?

main.cpp: In function 'constexpr void add()':

main.cpp:8:1: error: the value of 'd' is not usable in a constant expression

 }

 ^

main.cpp:4:5: note: 'int d' is not const

 int d = 1;

     ^