hardcore.cpp

//a.cpp
//a.* can not include b.*
struct Test {
    int Bar();
};

struct Test2 {
    int Barr();
};

template <class T>
int Foo(T& t) {
    return -1;
};

template <>
int Foo<Test>(Test& t) {
      return t.Bar();
}

template <>
int Foo<Test2>(Test2& t) {
      return t.Barr();
}

//b.cpp
#include "a.cpp"
#include <iostream>

using namespace std;

struct Test {
   int Bar() { return 1; }
};

struct Test2 {
   int Barr() { return 2; }
};

int main() {
    Test a;
    Test2 b;
    cout << Foo(a) << " " << Foo(b) << endl;
    return 0;
}