Py_IS_TYPE(): cast discards ‘const’ qualifier from pointer target type

[vstinner]

BPO	44378
Nosy	@vstinner, @miss-islington
PRs	bpo-44378: Fix a compiler warning in Py_IS_TYPE() #26644 [3.10] bpo-44378: Fix a compiler warning in Py_IS_TYPE() (GH-26644) #26668

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-06-11.09:44:59.760>
created_at = <Date 2021-06-10.13:19:54.087>
labels = ['expert-C-API', '3.10', '3.11']
title = 'Py_IS_TYPE(): cast discards \xe2\x80\x98const\xe2\x80\x99 qualifier from pointer target type'
updated_at = <Date 2021-06-11.09:44:59.760>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2021-06-11.09:44:59.760>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-11.09:44:59.760>
closer = 'vstinner'
components = ['C API']
creation = <Date 2021-06-10.13:19:54.087>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 44378
keywords = ['patch']
message_count = 4.0
messages = ['395534', '395535', '395617', '395622']
nosy_count = 2.0
nosy_names = ['vstinner', 'miss-islington']
pr_nums = ['26644', '26668']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44378'
versions = ['Python 3.10', 'Python 3.11']

[vstinner]

The following change introduced a compiler warning:

commit c5cb077
Author: Victor Stinner <vstinner@python.org>
Date: Tue Sep 22 12:42:28 2020 +0200

Py_IS_TYPE() macro uses Py_TYPE() (GH-22341)

Steps to Reproduce:

1. dnf install -y python3-devel pkgconf-pkg-config gcc
2. printf '#include <Python.h>' > test.c
3. gcc -Wcast-qual -Werror -c test.c pkg-config --cflags --libs python-3.10

Actual results:
sh-5.1# gcc -Wcast-qual -Werror -c test.c pkg-config --cflags --libs python-3.10
In file included from /usr/include/python3.10/Python.h:78,
from test.c:1:
/usr/include/python3.10/object.h: In function ‘_Py_IS_TYPE’:
/usr/include/python3.10/object.h:112:29: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
112 | #define _PyObject_CAST(op) ((PyObject*)(op))
| ^
/usr/include/python3.10/object.h:137:34: note: in expansion of macro ‘_PyObject_CAST’
137 | #define Py_TYPE(ob) (_PyObject_CAST(ob)->ob_type)
| ^~~~~~~~~~~~~~
/usr/include/python3.10/object.h:144:12: note: in expansion of macro ‘Py_TYPE’
144 | return Py_TYPE(ob) == type;
| ^~~~~~~
cc1: all warnings being treated as errors

Attached PR fix the compiler warning.

Issue reported on Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=1969663

[vstinner]

Comment from my PR:

// bpo-44378: Don't use Py_TYPE() since Py_TYPE() requires a non-const
// object.

By the way, I wrote a change in Python 3.11 to convert Py_TYPE() macro into a static inline function which accepts a *const* PyObject pointer ("const PyObject*"):
f3fa63e

But this change had to be reverted since it caused a buildbot regression (bpo-44348).

[vstinner]

New changeset 304dfec by Victor Stinner in branch 'main':
bpo-44378: Fix a compiler warning in Py_IS_TYPE() (GH-26644)
304dfec

[miss-islington]

New changeset e6d28a1 by Miss Islington (bot) in branch '3.10':
bpo-44378: Fix a compiler warning in Py_IS_TYPE() (GH-26644)
e6d28a1