On Darwin, the SDK is a directory of stubs, and libc++ lives separately. We
need to patch the CMake files in several places to make the build for C++
interop succeed. The required flags can be read from cc-wrapper support files.

--- a/SwiftCompilerSources/CMakeLists.txt
+++ b/SwiftCompilerSources/CMakeLists.txt
@@ -105,18 +105,11 @@ function(add_swift_compiler_modules_library name)
       get_filename_component(swift_exec_bin_dir ${ALS_SWIFT_EXEC} DIRECTORY)
       set(sdk_option ${sdk_option} "-resource-dir" "${swift_exec_bin_dir}/../bootstrapping0/lib/swift")
     endif()
-    if(NOT EXISTS "${sdk_path}/usr/include/c++")
-      # Darwin SDKs in Xcode 12 or older do not include libc++, which prevents clang from finding libc++ when invoked
-      # from ClangImporter. This results in build errors. To workaround this, let's explicitly pass the path to libc++
-      # to clang.
-      message(WARNING "Building with an outdated Darwin SDK: libc++ missing from the ${SWIFT_HOST_VARIANT_SDK} SDK. Will use libc++ from the toolchain.")
-      get_filename_component(absolute_libcxx_path "${CMAKE_C_COMPILER}/../../include/c++/v1" REALPATH)
-      if (EXISTS "${absolute_libcxx_path}")
-        set(sdk_option ${sdk_option} "-Xcc" "-isystem" "-Xcc" "${absolute_libcxx_path}")
-      else()
-        message(ERROR "libc++ not found in the toolchain.")
-      endif()
-    endif()
+    file(READ "$ENV{NIX_CC}/nix-support/libcxx-cxxflags" nix_libcxx_cxxflags)
+    separate_arguments(nix_libcxx_cxxflags)
+    foreach(nix_libcxx_cxxflag ${nix_libcxx_cxxflags})
+      set(sdk_option ${sdk_option} "-Xcc" "${nix_libcxx_cxxflag}")
+    endforeach()
   elseif(BOOTSTRAPPING_MODE STREQUAL "CROSSCOMPILE")
     set(sdk_option "-sdk" "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}")
     get_filename_component(swift_exec_bin_dir ${ALS_SWIFT_EXEC} DIRECTORY)
--- a/cmake/modules/SwiftConfigureSDK.cmake
+++ b/cmake/modules/SwiftConfigureSDK.cmake
@@ -270,6 +270,18 @@ macro(configure_sdk_darwin
   # Add this to the list of known SDKs.
   list(APPEND SWIFT_CONFIGURED_SDKS "${prefix}")
 
+  set(cxx_overlay_opt "")
+  if("${prefix}" STREQUAL "OSX")
+    file(READ "$ENV{NIX_CC}/nix-support/libcxx-cxxflags" nix_libcxx_cxxflags)
+    separate_arguments(nix_libcxx_cxxflags)
+    foreach(nix_libcxx_cxxflag ${nix_libcxx_cxxflags})
+      set(cxx_overlay_opt ${cxx_overlay_opt} "-Xcc" "${nix_libcxx_cxxflag}")
+    endforeach()
+  endif()
+  set(SWIFT_SDK_${prefix}_CXX_OVERLAY_SWIFT_COMPILE_FLAGS
+      ${cxx_overlay_opt}
+    CACHE STRING "Extra flags for compiling the C++ overlay")
+
   _report_sdk("${prefix}")
 endmacro()
 
--- a/stdlib/public/Cxx/std/CMakeLists.txt
+++ b/stdlib/public/Cxx/std/CMakeLists.txt
@@ -145,6 +145,9 @@ add_swift_target_library(swiftstd STATIC NO_LINK_NAME IS_STDLIB
     SWIFT_COMPILE_FLAGS_LINUX
     ${SWIFT_SDK_LINUX_CXX_OVERLAY_SWIFT_COMPILE_FLAGS}
 
+    SWIFT_COMPILE_FLAGS_OSX
+    ${SWIFT_SDK_OSX_CXX_OVERLAY_SWIFT_COMPILE_FLAGS}
+
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
     TARGET_SDKS ALL_APPLE_PLATFORMS LINUX
     INSTALL_IN_COMPONENT compiler