gh-99069: Consolidate checks for static_assert

[jmroot]

Several platforms don't define the static_assert macro despite having
compiler support for the _Static_assert keyword. The macro needs to be
defined since it is used unconditionally in the Python code. So it
should always be safe to define it if undefined and not in C++11 (or
later) mode.

Hence, remove the checks for particular platforms or libc versions,
and just define static_assert anytime it needs to be defined but isn't.
That way, all platforms that need the fix will get it, regardless of
whether someone specifically thought of them.

Also document that certain macOS versions are among the platforms that
need this.

• Issue: Build Python with C11 #91731

• Issue: static_assert undefined (with C11 compiler) on macOS <= 10.10 #99069

[jmroot]

@vstinner Is there something that is defined iff Python itself is being built? Other users of the headers don't necessarily need static_assert, so it would be nice to avoid defining it as an unexpected side effect. I guess that would allow removing the __cplusplus checks too, since Python won't be built as C++?

[vstinner]

Sadly, static_assert() is now part of the Python 3.11 C API. Removing it from its public C API in a 3.11.x bugfix release would be worse IMO.

[vstinner]

In practice, which platform/libc/compiler is impacted by this change? It would be nice to add a NEWS entry to discuss which platforms are affected.

[vstinner]

cc @pablogsal who modified this macro recently.

cc @ambv who may have an opinion on the macOS 10.10 support (issue #99069), purpose of this PR if I got it correctly

[jmroot]

In practice, which platform/libc/compiler is impacted by this change?

I only know of the ones mentioned in the comment, i.e. FreeBSD <= 12, macOS <= 10.10, and glibc < 2.16.

[vstinner]

I was very excited to be able to get rid of Py_BUILD_ASSERT() "hack" with a well defined (!) static_assert() function of the C11 standard. But month after month, I see that: weeeeeell, it depends, maybe it's available, maybe not. It depends on the libc, on the compiler, on the OS, etc.

Maybe we should give up with static_assert() and use _Static_assert(). Coming from Python, for me it's weird to use a name starting with an underscore :-( It reminds me non-standard APIs on Windows which looks like POSIX but are different, like _open().

Maybe Py_BUILD_ASSERT() is just better since it's battle tested (in Python) and it "just works" :-) I didn't expect that it would be so complicated to get static_assert() on all compilers on all platforms in 2022 :-(

A tradeoff might be to only use static_assert() inside Python and remove it from the public Python C API. Then the funny part is that depending if the proper (hypothetical) Python internal header file is included or not, using static_assert() might work or not. The problem is that some Python macros use assert(), so Python.h includes <assert.h>. But as you can see, static_assert() may or may not be available, Python currently has a complicated macro to make sure that it's always present. What if an internal Python C file uses <Python.h> but forgets to include "pycore_assert.h"? Maybe it works on all platforms. Maybe it fails on some specific platforms. Fun. Isn't it? That's why I put it in the public C API.

---

"Well defined": people are now discussing static_assert() with a single argument. I'm curious when it will be standard and when all compilers on all platforms will support it :-)

[jmroot]

@vstinner@pablogsal Is there a consensus for how you want to proceed here?

[vstinner]

@pablogsal asked to add __has_feature(c_static_assert) check. I don't see this check in your PR yet. Does it mean that you are against using it, and if yes, why?

[jmroot]

@vstinner Sorry if I misinterpreted your comments above; it looked to me like you were disagreeing with @pablogsal's proposal.

My own view is that a clang-specific check is not necessary, because the existence of _Static_assert does not depend on the stdlib at all, and all versions of clang that accept -std=c11 or -std=c1x provide that keyword. Restricting the define to clang and gcc would be reasonable though, just in case other compilers do something weird.

[jmroot]

@pablogsal Is this acceptable now?

[pablogsal]

Yup, this looks good to me. Thanks for working on this!

[vstinner]

For me, it's strange to check for GNUC with clang. The first code was added for FreeBSD which uses clang.

[jmroot]

@vstinner What is still needed for this to be ready to merge?

[miss-islington]

Thanks @jmroot for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-103282 is a backport of this pull request to the 3.11 branch.

[vstinner]

Merged, thanks.