[BUG] Cannot cythonize C++ source if function isn't explicitly used in a new Python function in the same file

[danilo-bc]

Describe the bug

This started as a Stack Overflow question and was pointed to be a possible bug.

Code to reproduce the behaviour:

hello.cpp

#include <iostream>

void hello_from_cpp() {
    std::cout << "Hello from C++!" << std::endl;
}

cpp_hello.pyx

# distutils: language = c++
# cython: language_level = 3
cpdef extern void hello_from_cpp()

def myfun():
    hello_from_cpp()

setup.py

from setuptools import setup, Extension
from Cython.Build import cythonize

setup(
    ext_modules=cythonize([
        Extension("cpp_hello", ["cpp_hello.pyx", "hello.cpp"])
        ]),
)

main.py

import cpp_hello

if __name__ == "__main__":
    cpp_hello.hello_from_cpp()

Built with python setup.py build_ext --inplace or python setup.py build_ext -i, tested with python main.py. The code compiles and presents the string in the terminal, but if the lines defining myfun in cpp_hello.pyx are removed, it fails to compile:

[1/1] Cythonizing cpp_hello.pyx
running build_ext
building 'cpp_hello' extension
creating build
creating build/temp.linux-x86_64-cpython-310
x86_64-linux-gnu-gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/usr/include/python3.10 -c cpp_hello.cpp -o build/temp.linux-x86_64-cpython-310/cpp_hello.o
cpp_hello.cpp: In function ‘int __pyx_pymod_exec_cpp_hello(PyObject*)’:
cpp_hello.cpp:3027:84: error: ‘hello_from_cpp’ was not declared in this scope
 3027 |     PyObject* wrapped = __Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(hello_from_cpp);
      |                                                                                    ^~~~~~~~~~~~~~
cpp_hello.cpp: At global scope:
cpp_hello.cpp:2362:18: warning: ‘PyObject* __Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(void (*)())’ defined but not used [-Wunused-function]
 2362 | static PyObject *__Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(void (*__pyx_v_f)(void)) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1

Expected behaviour

I should be able to expose the C++ function to Python using cpdef extern void hello_from_cpp() without requiring additional code.

OS

Linux/WSL2

Python version

3.10.12

Cython version

3.0.10

Additional context

I also tried the alternative declaration cdef extern from "hello.cpp", but I can't create a homonimous function hello_from_cpp() as I wish.