When is the address of a global variable passed as a template argument?

Question

I recently watched a video from Microsoft explaining some of the new features of C ++ 17, and I was intrigued to find a C ++ 14 function (and maybe even before that?) That allows you to use the address of a global variable as a constant expression for template argument.

This allows you to use the following code:

#include <iostream>
int g_iTest = 5;

template <int* Addr>
struct S {
    static int TestAdd( int iTest ) {
         *Addr = iTest + *Addr;
         return *Addr;
    }
};

int main() {
    S<&g_iTest> s;
    std::cout << s.TestAdd( 5 ) << std::endl;
}

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?

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