Why doesn't this "undefined extern variable" result in a linker error in C++17?

I have compiled and ran the following program in a C++17 compiler (Coliru). In the program, I declared an extern variable, but did not define it. However, the compiler doesn't give a linker error.

#include <iostream>

extern int i; // Only declaration

int func() 
{
    if constexpr (true)
        return 0;
    else if (i)
        return i;
    else
        return -1;
}

int main() 
{
    int ret = func();
    std::cout<<"Ret : "<<ret<<std::endl;
}

Why doesn't the compiler give a linker error?