Cython编译C++源码异常：未显式调用外部函数则编译失败

该问题已在Cython的bug追踪系统中记录。参考Cython调用C库教程尝试调用C++函数，在尝试类式语法定义外部函数失败后，找到一种编译方式，但出现异常行为。

测试涉及4个文件：

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()

使用python setup.py build_ext --inplace或python setup.py build_ext -i编译后，运行python main.py可正常输出Hello from C++!。

但移除cpp_hello.pyx中未使用的myfun()定义后，编译失败，报错如下：

[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

我的目标是创建可被Python调用的C代码库，通过.pyx和setup.py文件，用cpdef而非cdef定义C函数的Python接口。

内容的提问来源于stack exchange，提问作者DanBC