Reproducing "multiple declarations" error from linker

Question

So, I was going to demonstrate the problem of (non-template) implementation in C++ .h files to a college.

But right now I can't reproduce it as expected. Here's my code:

// common.h

#ifndef common_h
#define common_h

#include <iostream>

class common
{
    public:
    void Hello() {
        // Implementation in header file
        std::cout << "Hello from common" << std::endl;
    }
};

#endif // common_h

// user1.h

#ifndef user1_h
#define user1_h

#include "common.h"

class user1
{
    public:
    user1();
    
    private:
    common g;
};

#endif // user1_h

// user1.cpp

#include "user1.h"

#include <iostream>

user1::user1()
{
    std::cout << "Constructing user 1" << std::endl;
    g.Hello();
}

// user2.h

// Exactly as user1.h except the class+constructor name

// user2.cpp

// Exactly as user1.cpp except the class+constructor name

// main.cpp

#include "user1.h"
#include "user2.h"

int main()
{
    user1 u1;
    user2 u2;
}

I'm compiling the code like this:

g++ -c -g -O0 -fno-lto -fno-inline-small-functions user1.cpp
g++ -c -g -O0 -fno-lto -fno-inline-small-functions user2.cpp
g++ -c -g -O0 -fno-lto -fno-inline-small-functions main.cpp

g++ user1.o user2.o main.o

Nothing fails and program works as expected.

Why am I not getting a "multiple declarations" error on the "Hello" function in common.h?

I looked into the object files (using objdump) and I see this tag: LINK_ONCE_DISCARD. Could this be the reason? Can anyone supply more details? I'm sure I've seen this before so something must have change (maybe from a compiler update or something).

Compiler version:

>g++ --version
g++ (Rev2, Built by MSYS2 project) 12.1.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

Thanks to @BoP, here's the answer:

https://en.cppreference.com/w/cpp/language/inline

My "Hello" function is inline because it is defined inside a class declaration.