[llvm] [Bazel] Match LLVM MSVC conformance flags (PR #218187)

[Corentin Kerisit via llvm-commits]

https://github.com/cerisier created https://github.com/llvm/llvm-project/pull/218187

LLVM CMake enables /Zc:inline so conforming inline semantics permit unused COMDAT elimination, /Zc:__cplusplus so feature checks see the selected C++ language level, /permissive- so the frontend enforces conforming language rules, and /Zc:preprocessor so cl.exe uses the conforming preprocessor. Bazel omitted these flags, allowing the same LLVM sources to compile under different language and preprocessing rules. Apply the three shared flags to MSVC and clang-cl, keep /Zc:preprocessor restricted to cl.exe as CMake does, and keep /Zc:__cplusplus on C++ actions. Generic Windows and non-MSVC configurations remain unchanged.

>From 704ca40d4e8d8bb5f8ff621cfdbf5b942cf580b0 Mon Sep 17 00:00:00 2001
From: Corentin Kerisit <corentin.kerisit at gmail.com>
Date: Sun, 23 Aug 2026 01:32:00 +0900
Subject: [PATCH] [Bazel] Match CMake MSVC conformance flags

---
 utils/bazel/.bazelrc | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/utils/bazel/.bazelrc b/utils/bazel/.bazelrc
index d5c1352292e32..16930f636c763 100644
--- a/utils/bazel/.bazelrc
+++ b/utils/bazel/.bazelrc
@@ -158,6 +158,15 @@ build:windows --cxxopt=/bigobj --host_cxxopt=/bigobj
 
 build:msvc --config=windows
 
+# Match LLVM's default MSVC conformance flags. Unlike the other flags here,
+# /Zc:__cplusplus only applies to C++ compilations.
+build:msvc --copt=/Zc:inline --host_copt=/Zc:inline
+build:msvc --cxxopt=/Zc:__cplusplus --host_cxxopt=/Zc:__cplusplus
+build:msvc --copt=/permissive- --host_copt=/permissive-
+
+# CMake enables the conforming preprocessor for cl.exe, but not clang-cl.
+build:msvc --copt=/Zc:preprocessor --host_copt=/Zc:preprocessor
+
 build:msvc --copt=/WX --host_copt=/WX    # Treat warnings as errors...
 # ...but disable the ones that are violated
 build:msvc --copt=/wd4141 --host_copt=/wd4141 # inline used more than once
@@ -184,6 +193,12 @@ build:clang-cl --config=windows
 # Switch from MSVC to the `clang-cl` compiler.
 build:clang-cl --compiler=clang-cl
 
+# Match the conformance flags CMake applies to clang-cl. /Zc:preprocessor is
+# intentionally only part of the MSVC configuration above.
+build:clang-cl --copt=/Zc:inline --host_copt=/Zc:inline
+build:clang-cl --cxxopt=/Zc:__cplusplus --host_cxxopt=/Zc:__cplusplus
+build:clang-cl --copt=/permissive- --host_copt=/permissive-
+
 # Use Clang's internal warning flags instead of the ones that sometimes map
 # through to MSVC's flags.
 build:clang-cl --copt=/clang:-Wall --host_copt=/clang:-Wall