// Analysis of _Py_CAST() to debug:
// https://github.com/python/cpython/issues/92898
//
// g++ -Wall -Wextra -Wconversion -Wold-style-cast py_cast.cpp -o py_cast

#include <cassert>

struct PyObject
{
    int x;
};

struct PyLongObject
{
    PyObject ob_base;
    int size;
};

class StrongRef
{
public:
    StrongRef(PyObject *obj) : m_obj(obj) {
        //Py_INCREF(this->m_obj);
    }

    ~StrongRef() {
        //Py_DECREF(this->m_obj);
    }

    // Cast to PyObject*: get a borrowed reference
    inline operator PyObject*() const { return this->m_obj; }

private:
    PyObject *m_obj;  // Strong reference
};


#if 0
   // Old-style C cast: magic, it just works!
#  define _Py_CAST(type, expr) ((type)expr)

#elif 0
   // static_cast<>:
   // * PyObject* => PyObject*: ok
   // * const PyObject* => PyObject*: ERROR
   // * PyLongObject* => PyObject*: ERROR
   // * StrongRef => PyObject*: ok
#  define _Py_CAST(type, expr) static_cast<type>(expr)


#elif 0

   // reinterpret_cast<> + const_cast<>
   // * PyObject* => PyObject*: ok
   // * const PyObject* => PyObject*: ok
   // * PyLongObject* => PyObject*: ok
   // * StrongRef => PyObject*: ERROR
#  define _Py_CAST(type, expr) const_cast<type>(reinterpret_cast<const type>(expr))

#else

   // templates:
   //
   // PyObject* => PyObject*: ok
   // &PyObject => PyObject*: ok
   // PyLongObject* => PyObject*: ok
   // &PyLongObject => PyObject*: ok
   // const PyObject* => PyObject*: ok
   // &const PyObject => PyObject*: ok
   // StrongRef => PyObject*: ok
   //
   // https://github.com/python/cpython/pull/92138#issuecomment-1116983125

template<typename type, typename expr_type>
type _Py_reinterpret_cast_impl(expr_type* expr)  { return reinterpret_cast<type>(expr);}

template<typename type, typename expr_type>
type _Py_reinterpret_cast_impl(expr_type const * expr)  { return reinterpret_cast<type>(const_cast<expr_type*>(expr));}

template<typename type, typename expr_type>
type _Py_reinterpret_cast_impl(expr_type & expr)  { return static_cast<type>(expr);}

template<typename type, typename expr_type>
type _Py_reinterpret_cast_impl(expr_type const & expr)  { return static_cast<type>(const_cast<expr_type &>(expr));}

#  define _Py_CAST(type, expr) _Py_reinterpret_cast_impl<type>(expr)

#endif


int main()
{
    PyLongObject long_obj = {.ob_base = { .x = 1 }, .size = 2};
    PyObject obj = { .x = 3 };
    PyObject *obj_ptr = &obj;

    // PyObject* => PyObject*
    PyObject *cast1 = _Py_CAST(PyObject*, obj_ptr);
    assert(cast1 != nullptr);

    // &PyObject => PyObject*
    PyObject *cast1b = _Py_CAST(PyObject*, &obj);
    assert(cast1b != nullptr);

    // PyLongObject* => PyObject*
    PyLongObject *obj3 = &long_obj;
    PyObject *cast3 = _Py_CAST(PyObject*, obj3);
    assert(cast3 != nullptr);

    // &PyLongObject => PyObject*
    PyObject *cast3b = _Py_CAST(PyObject*, &long_obj);
    assert(cast3b != nullptr);

    // const PyObject* => PyObject*
    const PyObject *const_obj_ptr = obj_ptr;
    PyObject *cast2 = _Py_CAST(PyObject*, const_obj_ptr);
    assert(cast2 != nullptr);

    // &const PyObject => PyObject*
    const PyObject const_obj = obj;
    PyObject *cast2b = _Py_CAST(PyObject*, &const_obj);
    assert(cast2b != nullptr);

    // StrongRef => PyObject*
    StrongRef strong_ref(obj_ptr);
    PyObject *cast4 = _Py_CAST(PyObject*, strong_ref);
    assert(cast4 != nullptr);

    return 0;
}