[libcxx-commits] [libcxx] [libcxx] Mark system_reserved_names.gen.py unsupported on clang-20 && msvc (PR #161811)

[Martin Storsjö via libcxx-commits]

https://github.com/mstorsjo created https://github.com/llvm/llvm-project/pull/161811

Clang 20 (and early 21 versions; let's hope it can be fixed before the later versions before such versions become relevant for libcxx CI) have got an issue with its intrinsics headers, where they use unreserved names, that users are allowed to override.

See https://github.com/llvm/llvm-project/issues/161808 for the issue report.

This only crops up in the MSVC build configurations, as recent versions of some MSVC/UCRT headers include `<intrin.h>`, which ends up pulling in most intrinsics headers, exposing this issue in the Clang headers.

This should unblock https://github.com/llvm/llvm-project/pull/161736 from being merged.

>From b37d2cb479b0258e1c3a0c3f9bb2e5d24aa259cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin at martin.st>
Date: Fri, 3 Oct 2025 11:51:24 +0300
Subject: [PATCH] [libcxx] Mark system_reserved_names.gen.py unsupported on
 clang-20 && msvc

Clang 20 (and early 21 versions; let's hope it can be fixed
before the later versions before such versions become relevant
for libcxx CI) have got an issue with its intrinsics headers,
where they use unreserved names, that users are allowed to override.

See https://github.com/llvm/llvm-project/issues/161808 for the
issue report.

This only crops up in the MSVC build configurations, as recent
versions of some MSVC/UCRT headers include `<intrin.h>`, which
ends up pulling in most intrinsics headers, exposing this issue
in the Clang headers.

This should unblock https://github.com/llvm/llvm-project/pull/161736
from being merged.
---
 libcxx/test/libcxx/system_reserved_names.gen.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libcxx/test/libcxx/system_reserved_names.gen.py b/libcxx/test/libcxx/system_reserved_names.gen.py
index f8589f2beaae3..d69182d68e0de 100644
--- a/libcxx/test/libcxx/system_reserved_names.gen.py
+++ b/libcxx/test/libcxx/system_reserved_names.gen.py
@@ -10,6 +10,13 @@
 # alphabetic macros. Also ensure that we don't swallow the definition of user
 # provided macros (in other words, ensure that we push/pop correctly everywhere).
 
+# This test fails with MSVC headers, with Clang 20 (and early 21 versions);
+# the headers end up pulling in Clang intrinsics headers, which in 20.x and
+# early 21.x versions use unreserved identifiers,
+# see https://github.com/llvm/llvm-project/issues/161808.
+#
+# UNSUPPORTED: clang-20 && msvc
+
 # RUN: %{python} %s %{libcxx-dir}/utils
 # END.