A simple project where IWYU fails

container.cpp

#include "container.h"

IMPLEMENTATION_DEFINED_CONTAINER_TYPE container;

container.h

#pragma once

// IMPLEMENTATION_DEFINED_CONTAINER_TYPE is guaranteed to fulfill some concept
// (say, SequenceContainer<int>)
#ifdef USE_DEQUE
#include <deque>
using IMPLEMENTATION_DEFINED_CONTAINER_TYPE = std::deque<int>;
#else
#include <vector>
using IMPLEMENTATION_DEFINED_CONTAINER_TYPE = std::vector<int>;
#endif

extern IMPLEMENTATION_DEFINED_CONTAINER_TYPE container;

main.cpp

#include "container.h"
#include <iostream>

int main() {
    container.push_back(5);
    auto it = container.begin();
    std::cout << *it;
}