From bd0dd33de11c7905028f6521a54a8e99e8de4d94 Mon Sep 17 00:00:00 2001
From: Randy Eckenrode <randy@largeandhighquality.com>
Date: Mon, 21 Apr 2025 18:51:52 -0400
Subject: [PATCH 1/2] cc-wrapper: always include libc++ in the search path

The libc++ headers are expected to be found in the sysroot when `clang`
is invoked for C++ code.

Always making them available improves compatibility with build systems
like Bazel and SwiftPM, which try to compile C++ code with `clang`.
---
 pkgs/build-support/cc-wrapper/cc-wrapper.sh | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/pkgs/build-support/cc-wrapper/cc-wrapper.sh b/pkgs/build-support/cc-wrapper/cc-wrapper.sh
index eb5b99643dbac..246c1b1f65f6e 100644
--- a/pkgs/build-support/cc-wrapper/cc-wrapper.sh
+++ b/pkgs/build-support/cc-wrapper/cc-wrapper.sh
@@ -140,6 +140,15 @@ if [ "$NIX_ENFORCE_NO_NATIVE_@suffixSalt@" = 1 ]; then
     params=(${kept+"${kept[@]}"})
 fi
 
+# Some build systems such as Bazel and SwiftPM use `clang` instead of `clang++`,
+# which will find the libc++ headers in the sysroot for C++ files.
+if [[ "$isCxx" = 0 && "@isClang@" ]]; then
+# This duplicates the behavior of a native toolchain, which can find the
+# libc++ headers but requires `-lc++` to be specified explicitly when linking.
+    isCxx=1
+    cxxLibrary=0
+fi
+
 if [[ "$isCxx" = 1 ]]; then
     if [[ "$cxxInclude" = 1 ]]; then
         #

From 76a8ffa6d8d2f93b81d4e9be327a01cab1c893c0 Mon Sep 17 00:00:00 2001
From: Randy Eckenrode <randy@largeandhighquality.com>
Date: Mon, 22 Sep 2025 18:53:09 -0400
Subject: [PATCH 2/2] cc-wrapper: use `-cxx-isystem` instead of `-isystem` on
 Clang

This allows Clang to always look for the libc++ headers when compling
C++ code (regardless of whether it is invoked with `clang` or `clang++`
while not exposing the headers when compiling plain C code.

See https://releases.llvm.org/21.1.0/tools/clang/docs/ClangCommandLineReference.html#cmdoption-clang-cxx-isystem-directory
---
 pkgs/build-support/cc-wrapper/default.nix | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pkgs/build-support/cc-wrapper/default.nix b/pkgs/build-support/cc-wrapper/default.nix
index 50ff90609a5e4..427161f4742c0 100644
--- a/pkgs/build-support/cc-wrapper/default.nix
+++ b/pkgs/build-support/cc-wrapper/default.nix
@@ -741,14 +741,14 @@ stdenvNoCC.mkDerivation {
     # https://github.com/NixOS/nixpkgs/pull/209870#issuecomment-1500550903)
     + optionalString (libcxx == null && isClang && (useGccForLibs && gccForLibs.langCC or false)) ''
       for dir in ${gccForLibs}/include/c++/*; do
-        include -isystem "$dir" >> $out/nix-support/libcxx-cxxflags
+        include -cxx-isystem "$dir" >> $out/nix-support/libcxx-cxxflags
       done
       for dir in ${gccForLibs}/include/c++/*/${targetPlatform.config}; do
-        include -isystem "$dir" >> $out/nix-support/libcxx-cxxflags
+        include -cxx-isystem "$dir" >> $out/nix-support/libcxx-cxxflags
       done
     ''
     + optionalString (libcxx.isLLVM or false) ''
-      include -isystem "${getDev libcxx}/include/c++/v1" >> $out/nix-support/libcxx-cxxflags
+      include -cxx-isystem "${getDev libcxx}/include/c++/v1" >> $out/nix-support/libcxx-cxxflags
       echo "-stdlib=libc++" >> $out/nix-support/libcxx-ldflags
     ''
     # GCC NG friendly libc++