Understanding extern templates in c++

I am trying to understand extern templates, but i can't get it to work. My goal is to compile some instanciations if Foo<> in a seperate compilation unit in order to save compilation time. In my code base, the template argument is a enum class, so in theory I am able to compile all instanciations in a compilation unit and link them with the rest of the project.

Here is a little example I come up with:

//Foo.hpp
#pragma once

template <class T>
struct Foo {
    Foo();
    ~Foo();
};

extern template struct Foo<int>;

---

//Foo.cpp
#include "Foo.hpp"

template struct Foo<int>;

---

//main.cpp
#include <iostream>

#include "Foo.hpp"

int main(int argc, char **argv) {
   Foo<int> foo;

    return 0;
}

To for compilation i used a makefile, that boils down to this:

g++ -c ../Foo.cpp ../main.cpp
g++ Foo.o main.o 

The output I get with gcc 7.1.1 and basically the same with clang 4.0.1 is:

main.o: In function `main':
main.cpp:(.text+0x27): undefined reference to `Foo<int>::Foo()'
main.cpp:(.text+0x38): undefined reference to `Foo<int>::~Foo()'

My main question is, why is Foo<int>::Foo() and Foo<int>::~Foo()not compiled into Foo.o?