clang: skip the -nostdlibinc patch on Darwin

pkgs/development/compilers/llvm/common/clang/default.nix

@@ -81,13 +81,17 @@ let
       # Make sure clang passes the correct location of libLTO to ld64
       substituteInPlace lib/Driver/ToolChains/Darwin.cpp \
         --replace-fail 'StringRef P = llvm::sys::path::parent_path(D.Dir);' 'StringRef P = "${lib.getLib libllvm}";'
-    '' + (if lib.versionOlder release_version "13" then ''
-      sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
-             -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
-             lib/Driver/ToolChains/*.cpp
-    '' else ''
-      (cd tools && ln -s ../../clang-tools-extra extra)
-    '') + lib.optionalString stdenv.hostPlatform.isMusl ''
+    '' + (
+      # See the comment on the `add-nostdlibinc-flag.patch` patch in
+      # `../default.nix` for why we skip Darwin here.
+      if lib.versionOlder release_version "13" && (!stdenv.hostPlatform.isDarwin || !stdenv.targetPlatform.isDarwin) then ''
+        sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
+               -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
+               lib/Driver/ToolChains/*.cpp
+      '' else ''
+        (cd tools && ln -s ../../clang-tools-extra extra)
+      ''
+    ) + lib.optionalString stdenv.hostPlatform.isMusl ''
       sed -i -e 's/lgcc_s/lgcc_eh/' lib/Driver/ToolChains/*.cpp
     '';
 

pkgs/development/compilers/llvm/common/default.nix

@@ -529,8 +529,24 @@ let
             # mis-compilation in firefox.
             # See: https://bugzilla.mozilla.org/show_bug.cgi?id=1741454
             (metadata.getVersionFile "clang/revert-malloc-alignment-assumption.patch")
+          # This patch prevents global system header directories from
+          # leaking through on non‐NixOS Linux. However, on macOS, the
+          # SDK path is used as the sysroot, and forcing `-nostdlibinc`
+          # breaks `-isysroot` with an unwrapped compiler. As macOS has
+          # no `/usr/include`, there’s essentially no risk to skipping
+          # the patch there. It’s possible that Homebrew headers in
+          # `/usr/local/include` might leak through to unwrapped
+          # compilers being used without an SDK set or something, but
+          # it hopefully shouldn’t matter.
+          #
+          # TODO: Figure out a better solution to this whole problem so
+          # that we won’t have to choose between breaking unwrapped
+          # compilers breaking libclang when we can do Linux‐to‐Darwin
+          # cross‐compilation again.
+          ++ lib.optional (
+            !args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin
+          ) ./clang/add-nostdlibinc-flag.patch
           ++ [
-            ./clang/add-nostdlibinc-flag.patch
             (substituteAll {
               src =
                 if (lib.versionOlder metadata.release_version "16") then