system header warning

main.cpp

// not system include
//   gcc: https://wandbox.org/permlink/MLrLASaM5o5A3UWm
//   clang: https://wandbox.org/permlink/x6TT2NFROSmyLhV4
// system include
//   gcc: https://wandbox.org/permlink/XVHXnWIMyOfDMjl3
//   clang: https://wandbox.org/permlink/strTYwduT4ZLH5NK

// This file is a "Hello, world!" in C++ language by GCC for wandbox.
#include <iostream>
#include <cstdlib>
#include "test.h"
//#include <test.h>

int main()
{
    std::cout << "Hello, Wandbox!" << f() << std::endl;
}

// GCC reference:
//   https://gcc.gnu.org/

// C++ language references:
//   https://cppreference.com/
//   https://isocpp.org/
//   http://www.open-std.org/jtc1/sc22/wg21/

// Boost libraries references:
//   https://www.boost.org/doc/

test.h

int f() {
    return 1, 2;
}