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.

2 files changed, 44 insertions, 5 deletions

diff --git a/Lib/test/_testcppext.cpp b/Lib/test/_testcppext.cpp
index 14cd1dd..257843b 100644
--- a/Lib/test/_testcppext.cpp
+++ b/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 */
 };
 
diff --git a/Lib/test/test_cppext.py b/Lib/test/test_cppext.py
index c1d02bc..99f6062 100644
--- a/Lib/test/test_cppext.py
+++ b/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 @@ class TestCPPExt(unittest.TestCase):
             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")