gh-92135: Fix _Py_reinterpret_cast() for const (#92138)

Fix C++ compiler warnings on cast macros, like _PyObject_CAST(), when
casting a constant expression to a non constant type: use
const_cast<> in C++.

* In C++, Py_SAFE_DOWNCAST() now uses static_cast<> rather than
  reinterpret_cast<>.
* Add tests to the _testcppext C++ extension.
* test_cppext no longer captures stdout in verbose mode.

Include/methodobject.h

@@ -43,8 +43,7 @@ typedef PyObject *(*PyCMethod)(PyObject *, PyTypeObject *, PyObject *const *,
 // it triggers an undefined behavior when Python calls it with 2 parameters
 // (bpo-33012).
 #define _PyCFunction_CAST(func) \
-    _Py_reinterpret_cast(PyCFunction, \
-        _Py_reinterpret_cast(void(*)(void), (func)))
+    _Py_reinterpret_cast(PyCFunction, _Py_reinterpret_cast(void(*)(void), (func)))
 
 PyAPI_FUNC(PyCFunction) PyCFunction_GetFunction(PyObject *);
 PyAPI_FUNC(PyObject *) PyCFunction_GetSelf(PyObject *);

Include/objimpl.h

@@ -182,9 +182,9 @@ PyAPI_FUNC(void) PyObject_GC_UnTrack(void *);
 PyAPI_FUNC(void) PyObject_GC_Del(void *);
 
 #define PyObject_GC_New(type, typeobj) \
-                _Py_reinterpret_cast(type*, _PyObject_GC_New(typeobj))
+    _Py_reinterpret_cast(type*, _PyObject_GC_New(typeobj))
 #define PyObject_GC_NewVar(type, typeobj, n) \
-                _Py_reinterpret_cast(type*, _PyObject_GC_NewVar((typeobj), (n)))
+    _Py_reinterpret_cast(type*, _PyObject_GC_NewVar((typeobj), (n)))
 
 PyAPI_FUNC(int) PyObject_GC_IsTracked(PyObject *);
 PyAPI_FUNC(int) PyObject_GC_IsFinalized(PyObject *);

Include/pyport.h

@@ -14,10 +14,20 @@
 #endif
 
 
-// Macro to use C++ static_cast<> and reinterpret_cast<> in the Python C API
+// Macro to use C++ static_cast<>, reinterpret_cast<> and const_cast<>
+// in the Python C API.
+//
+// In C++, _Py_reinterpret_cast(type, expr) converts a constant expression to a
+// non constant type using const_cast<type>. For example,
+// _Py_reinterpret_cast(PyObject*, op) can convert a "const PyObject*" to
+// "PyObject*".
+//
+// The type argument must not be constant. For example, in C++,
+// _Py_reinterpret_cast(const PyObject*, expr) fails with a compiler error.
 #ifdef __cplusplus
 #  define _Py_static_cast(type, expr) static_cast<type>(expr)
-#  define _Py_reinterpret_cast(type, expr) reinterpret_cast<type>(expr)
+#  define _Py_reinterpret_cast(type, expr) \
+       const_cast<type>(reinterpret_cast<const type>(expr))
 #else
 #  define _Py_static_cast(type, expr) ((type)(expr))
 #  define _Py_reinterpret_cast(type, expr) ((type)(expr))
@@ -307,10 +317,10 @@ extern "C" {
  */
 #ifdef Py_DEBUG
 #  define Py_SAFE_DOWNCAST(VALUE, WIDE, NARROW) \
-       (assert((WIDE)(NARROW)(VALUE) == (VALUE)), (NARROW)(VALUE))
+       (assert(_Py_static_cast(WIDE, _Py_static_cast(NARROW, (VALUE))) == (VALUE)), \
+        _Py_static_cast(NARROW, (VALUE)))
 #else
-#  define Py_SAFE_DOWNCAST(VALUE, WIDE, NARROW) \
-       _Py_reinterpret_cast(NARROW, (VALUE))
+#  define Py_SAFE_DOWNCAST(VALUE, WIDE, NARROW) _Py_static_cast(NARROW, (VALUE))
 #endif
 
 

Lib/test/_testcppext.cpp

@@ -1,6 +1,9 @@
 // gh-91321: Very basic C++ test extension to check that the Python C API is
 // compatible with C++ and does not emit C++ compiler warnings.
 
+// Always enable assertions
+#undef NDEBUG
+
 #include "Python.h"
 
 PyDoc_STRVAR(_testcppext_add_doc,
@@ -20,8 +23,36 @@ _testcppext_add(PyObject *Py_UNUSED(module), PyObject *args)
 }
 
 
+static PyObject *
+test_api_casts(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    PyObject *obj = Py_BuildValue("(ii)", 1, 2);
+    if (obj == nullptr) {
+        return nullptr;
+    }
+
+    // gh-92138: For backward compatibility, functions of Python C API accepts
+    // "const PyObject*". Check that using it does not emit C++ compiler
+    // warnings.
+    const PyObject *const_obj = obj;
+    Py_INCREF(const_obj);
+    Py_DECREF(const_obj);
+    PyTypeObject *type = Py_TYPE(const_obj);
+    assert(Py_REFCNT(const_obj) >= 1);
+
+    assert(type == &PyTuple_Type);
+    assert(PyTuple_GET_SIZE(const_obj) == 2);
+    PyObject *one = PyTuple_GET_ITEM(const_obj, 0);
+    assert(PyLong_AsLong(one) == 1);
+
+    Py_DECREF(obj);
+    Py_RETURN_NONE;
+}
+
+
 static PyMethodDef _testcppext_methods[] = {
     {"add", _testcppext_add, METH_VARARGS, _testcppext_add_doc},
+    {"test_api_casts", test_api_casts, METH_NOARGS, NULL},
     {nullptr, nullptr, 0, nullptr}  /* sentinel */
 };
 

Lib/test/test_cppext.py

@@ -1,5 +1,6 @@
 # gh-91321: Build a basic C++ test extension to check that the Python C API is
 # compatible with C++ and does not emit C++ compiler warnings.
+import contextlib
 import os
 import sys
 import unittest
@@ -39,17 +40,24 @@ def build(self):
             sources=[SOURCE],
             language='c++',
             extra_compile_args=CPPFLAGS)
+        capture_stdout = (not support.verbose)
 
         try:
             try:
-                with (support.captured_stdout() as stdout,
-                      support.swap_attr(sys, 'argv', ['setup.py', 'build_ext'])):
+                if capture_stdout:
+                    stdout = support.captured_stdout()
+                else:
+                    print()
+                    stdout = contextlib.nullcontext()
+                with (stdout,
+                      support.swap_attr(sys, 'argv', ['setup.py', 'build_ext', '--verbose'])):
                     setup(name="_testcppext", ext_modules=[cpp_ext])
                     return
             except:
-                # Show output on error
-                print()
-                print(stdout.getvalue())
+                if capture_stdout:
+                    # Show output on error
+                    print()
+                    print(stdout.getvalue())
                 raise
         except SystemExit:
             self.fail("Build failed")