https://github.com/youngd007 created
https://github.com/llvm/llvm-project/pull/213045
Several Windows headers/TUs unconditionally `#define WIN32_LEAN_AND_MEAN`. When
a build predefines the macro on the compiler command line (e.g. a toolchain
that passes -DWIN32_LEAN_AND_MEAN, which clang treats as `#define
WIN32_LEAN_AND_MEAN 1`), the differing token lists trigger -Wmacro-redefined,
which becomes a hard error under -Werror.
Guard the definitions with #ifndef, matching the adjacent NOMINMAX handling and
the existing pattern in llvm/lib/Support/rpmalloc/rpmalloc.c. The macro is only
a presence flag, so keeping an externally-provided definition is correct.
Sites guarded:
llvm/include/llvm/Support/Windows/WindowsSupport.h
llvm/lib/WindowsDriver/MSVCPaths.cpp
clang/lib/Driver/ToolChains/MSVC.cpp
lldb/include/lldb/Host/windows/windows.h
Meta ran into this internally building lldb on window after a sync with
upstream from July commit, so adding guards would allow us to drop a workaround
of ignoring the duplicate defines.
Error:
llvm\include\llvm/Support/Windows/WindowsSupport.h(29,9): error:
'WIN32_LEAN_AND_MEAN' macro redefined [-Werror,-Wmacro-redefined]
29 | #define WIN32_LEAN_AND_MEAN
| ^
<command line>(10,9): note: previous definition is here
10 | #define WIN32_LEAN_AND_MEAN 1
| ^
1 error generated.
>From 13dcd6e0dd53af694baa06bbfee299a34b3cdacb Mon Sep 17 00:00:00 2001
From: David Young <[email protected]>
Date: Thu, 30 Jul 2026 08:07:05 -0700
Subject: [PATCH] [Support][Driver][LLDB] Guard WIN32_LEAN_AND_MEAN definitions
with #ifndef
Several Windows headers/TUs unconditionally `#define WIN32_LEAN_AND_MEAN`.
When a build predefines the macro on the compiler command line (e.g. a
toolchain that passes -DWIN32_LEAN_AND_MEAN, which clang treats as
`#define WIN32_LEAN_AND_MEAN 1`), the differing token lists trigger
-Wmacro-redefined, which becomes a hard error under -Werror.
Guard the definitions with #ifndef, matching the adjacent NOMINMAX handling
and the existing pattern in llvm/lib/Support/rpmalloc/rpmalloc.c. The macro
is only a presence flag, so keeping an externally-provided definition is
correct.
Sites guarded:
llvm/include/llvm/Support/Windows/WindowsSupport.h
llvm/lib/WindowsDriver/MSVCPaths.cpp
clang/lib/Driver/ToolChains/MSVC.cpp
lldb/include/lldb/Host/windows/windows.h
---
clang/lib/Driver/ToolChains/MSVC.cpp | 2 ++
lldb/include/lldb/Host/windows/windows.h | 2 ++
llvm/include/llvm/Support/Windows/WindowsSupport.h | 2 ++
llvm/lib/WindowsDriver/MSVCPaths.cpp | 2 ++
4 files changed, 8 insertions(+)
diff --git a/clang/lib/Driver/ToolChains/MSVC.cpp
b/clang/lib/Driver/ToolChains/MSVC.cpp
index 07ad9215fc029..07cf2ee2122f3 100644
--- a/clang/lib/Driver/ToolChains/MSVC.cpp
+++ b/clang/lib/Driver/ToolChains/MSVC.cpp
@@ -26,7 +26,9 @@
#include <cstdio>
#ifdef _WIN32
+ #ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
+ #endif
#define NOGDI
#ifndef NOMINMAX
#define NOMINMAX
diff --git a/lldb/include/lldb/Host/windows/windows.h
b/lldb/include/lldb/Host/windows/windows.h
index d53d4b9967268..bfaafdc952585 100644
--- a/lldb/include/lldb/Host/windows/windows.h
+++ b/lldb/include/lldb/Host/windows/windows.h
@@ -12,7 +12,9 @@
#define NTDDI_VERSION NTDDI_VISTA
#undef _WIN32_WINNT // undef a previous definition to avoid warning
#define _WIN32_WINNT _WIN32_WINNT_VISTA
+#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
+#endif
#define NOGDI
#undef NOMINMAX // undef a previous definition to avoid warning
#define NOMINMAX
diff --git a/llvm/include/llvm/Support/Windows/WindowsSupport.h
b/llvm/include/llvm/Support/Windows/WindowsSupport.h
index 50a2540dba687..fe475f0b629ce 100644
--- a/llvm/include/llvm/Support/Windows/WindowsSupport.h
+++ b/llvm/include/llvm/Support/Windows/WindowsSupport.h
@@ -26,7 +26,9 @@
// Require at least Windows 7 API.
#define _WIN32_WINNT 0x0601
+#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
+#endif
#ifndef NOMINMAX
#define NOMINMAX
#endif
diff --git a/llvm/lib/WindowsDriver/MSVCPaths.cpp
b/llvm/lib/WindowsDriver/MSVCPaths.cpp
index 09468dab10260..3092ee2fbf5b9 100644
--- a/llvm/lib/WindowsDriver/MSVCPaths.cpp
+++ b/llvm/lib/WindowsDriver/MSVCPaths.cpp
@@ -27,7 +27,9 @@
#endif
#ifdef _WIN32
+#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
+#endif
#define NOGDI
#ifndef NOMINMAX
#define NOMINMAX
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