https://github.com/rorth created 
https://github.com/llvm/llvm-project/pull/212716

`clang++` defaults to `-stdlib=libc++` on NetBSD.  When building with both 
`clang` and `libcxx` included, the freshly built `clang++` fails to find 
`<__config_site>`:

```
In file included from /usr/include/strings.h:68:
In file included from bin/../include/c++/v1/string.h:57:
bin/../include/c++/v1/__config:13:10: fatal
error: '__config_site' file not found
   13 | #include <__config_site>
      |          ^~~~~~~~~~~~~~~
```

The file is present in `include/<triplet>/c++/v1`, but that isn't searched by 
default.  NetBSD has its own version of addLibCxxIncludePaths which misses that 
directory.

Rather than replicating yet another part of the generic version in `Gnu.cpp`, 
this patch just calls `Generic_GCC::addLibCxxIncludePaths` from there.  It 
keeps to also include `/usr/include/c++`, although this directory only contains 
empty directories in my installation.

Tested on `amd64-pc-netbsd10.1`.

I know that this patch also needs testcases (currently there are none that are 
affected by this change), but I'd like to get this into the system first to 
check that the approach is sound.

>From b83adc46e93341e46eec64c1b4396181816d27aa Mon Sep 17 00:00:00 2001
From: Rainer Orth <[email protected]>
Date: Wed, 29 Jul 2026 10:52:08 +0200
Subject: [PATCH] [clang][Driver] Fix libc++ include path on NetBSD

`clang++` defaults to `-stdlib=libc++` on NetBSD.  When building with both
`clang` and `libcxx` included, the freshly built `clang++` fails to find
`<__config_site>`:

```
In file included from /usr/include/strings.h:68:
In file included from bin/../include/c++/v1/string.h:57:
bin/../include/c++/v1/__config:13:10: fatal
error: '__config_site' file not found
   13 | #include <__config_site>
      |          ^~~~~~~~~~~~~~~
```

The file is present in `include/<triplet>/c++/v1`, but that isn't searched
by default.  NetBSD has its own version of addLibCxxIncludePaths which
misses that directory.

Rather than replicating yet another part of the generic version in
`Gnu.cpp`, this patch just calls `Generic_GCC::addLibCxxIncludePaths` from
there.  It keeps to also include `/usr/include/c++`, although this
directory only contains empty directories in my installation.

Tested on `amd64-pc-netbsd10.1`.

I know that this patch also needs testcases (currently there are none that
are affected by this change), but I'd like to get this into the system
first to check that the approach is sound.
---
 clang/lib/Driver/ToolChains/NetBSD.cpp | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

diff --git a/clang/lib/Driver/ToolChains/NetBSD.cpp 
b/clang/lib/Driver/ToolChains/NetBSD.cpp
index 31a5723c17c2f..186a34143da37 100644
--- a/clang/lib/Driver/ToolChains/NetBSD.cpp
+++ b/clang/lib/Driver/ToolChains/NetBSD.cpp
@@ -6,6 +6,7 @@
 //
 
//===----------------------------------------------------------------------===//
 
+#include "Gnu.h"
 #include "NetBSD.h"
 #include "Arch/ARM.h"
 #include "Arch/Mips.h"
@@ -497,23 +498,14 @@ void NetBSD::AddClangSystemIncludeArgs(
 
 void NetBSD::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args) const {
-  const std::string Candidates[] = {
-    // directory relative to build tree
-    concat(getDriver().Dir, "/../include/c++/v1"),
-    // system install with full upstream path
-    concat(getDriver().SysRoot, "/usr/include/c++/v1"),
-    // system install from src
-    concat(getDriver().SysRoot, "/usr/include/c++"),
-  };
-
-  for (const auto &IncludePath : Candidates) {
-    if (!getVFS().exists(IncludePath + "/__config"))
-      continue;
-
-    // Use the first candidate that looks valid.
+  Generic_GCC::addLibCxxIncludePaths(DriverArgs, CC1Args);
+
+  // system install from src
+  const std::string IncludePath =
+      concat(getDriver().SysRoot, "/usr/include/c++");
+
+  if (getVFS().exists(IncludePath + "/__config"))
     addSystemInclude(DriverArgs, CC1Args, IncludePath);
-    return;
-  }
 }
 
 void NetBSD::addLibStdCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,

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