void print(const base&);

// header 1
struct base {};
void print(const base&);

// header 2
#include "header 1"
struct derived:base{};

// header 3
struct derived;
void print(const derived&);

// source 1
#include "header 2"

void foo() {
    derived d;
    print(d);
}

// source 2
#include "header 2"
#include "header 3"

void bar() {
    derived d;
    print(d);
}