Why does this usage of c++17 if constexpr fail?

I'm trying to use c++17 if constexpr for conditionally compilation but it does not behave the way I expect.

For example, with the code below, c++ still compiles the code defined by the macro X2,

#include <string>
#include <iostream>
#include <type_traits>

#define X1 pp("x")
#define X2 pp("x", "x")

void pp(const std::string str){
    std::cout << str << std::endl;
}

int main(){
    std::map<std::string, int> map;

    if constexpr (std::is_null_pointer_v<decltype(map)>)
        X2;
    else
        X1;
}

and spits this error messages:

1.c:6:23: error: too many arguments to function ‘void pp(std::__cxx11::string)’
 #define X2 pp("x", "x")
                       ^
1.c:18:3: note: in expansion of macro ‘X2’
   X2;
   ^~

Any idea how to skip compilation of X2?

Thanks!