CMake "OBJECT" library: clang not linking properly - c++

Question

I have a shared library (which currently compiles, loads and runs) mylib.so. From within this library, I want to use a new function (register it in another external library). The signature is bool my_function(const QVariant *, PyObject **).
This new function is defined in a separate .cpp file which is compiled to an object and then linked to mylib.so.
So I create a new OBJECT with my custom function
ADD_LIBRARY(helper_lib OBJECT helper_lib.cpp)
And include this when building my library
ADD_LIBRARY(mylib SHARED source.cpp $<TARGET_OBJECTS:helper_lib>)
It fails with an "undefined reference to `my_function'"
I can see that
The helper_lib.o file is generated
nm helper_lib.o shows
0000000000000000 T _Z11my_functionPK8QVariantPP7_object
nm mylib.o shows
U my_function
The helper_lib.o is passed to clang++ :
clang++ -fPIC [...] -o my_lib.so mylib.o helper_lib.o [...]
I struggle to see where the mistake happens. I can imagine that there is something wrong in mylib.o which shows an unmangled symbol name which cannot be matched to the helper_lib.o symbol name, but I may as well be totally on the wrong track with this.
helper_lib.h
void my_function();
helper_lib.cpp
#include "helper_lib.h"
void my_function()
{
return;
}
source.cpp is more complicated, as it contains mainly code automatically generated by sip.

Answer 1

It works for me with a simple source.cpp. So it must be that something gets messed up during inclusion, you can try moving #include "helper_lib.h to the top of your source.cpp.
To verify that this has nothing to do with your toolchain, you can try from a clean build directory the following project:
CMakeLists.txt:
cmake_minimum_required(VERSION 3.3)
project(dummy)
ADD_LIBRARY(helper_lib OBJECT helper_lib.cpp)
ADD_LIBRARY(mylib SHARED source.cpp $<TARGET_OBJECTS:helper_lib>)
source.cpp:
#include "helper_lib.h"
void dummy() {
my_function();
}
helper_lib.h:
#pragma once
void my_function();
helper_lib.cpp:
#include "helper_lib.h"
void my_function() {
}
Some documentation.