diff --git a/stdlib/private/CMakeLists.txt b/stdlib/private/CMakeLists.txt
index dbd63246a0396..23aef38b732b5 100644
--- a/stdlib/private/CMakeLists.txt
+++ b/stdlib/private/CMakeLists.txt
@@ -2,23 +2,16 @@ if(SWIFT_BUILD_STDLIB OR SWIFT_BUILD_SDK_OVERLAY)
   add_subdirectory(SwiftPrivate)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
-  if(SWIFT_BUILD_SDK_OVERLAY)
-    # FIXME: there is nothing Darwin-specific in StdlibUnittest, but to use
-    # POSIX APIs it imports the Darwin module on Apple platforms, so it can't
-    # be built separately from the SDK overlay.
-    add_subdirectory(StdlibUnittest)
-    add_subdirectory(StdlibCollectionUnittest)
-    add_subdirectory(StdlibUnittestFoundationExtras)
-    add_subdirectory(SwiftPrivateLibcExtras)
-    add_subdirectory(SwiftPrivatePthreadExtras)
-    add_subdirectory(SwiftReflectionTest)
-  endif()
-endif()
-
-if(CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+if(SWIFT_BUILD_SDK_OVERLAY)
+  # SwiftPrivatePthreadExtras makes use of Darwin/Glibc, which is part of the
+  # SDK overlay. It can't be built separately from the SDK overlay.
   add_subdirectory(StdlibUnittest)
   add_subdirectory(StdlibCollectionUnittest)
   add_subdirectory(SwiftPrivateLibcExtras)
   add_subdirectory(SwiftPrivatePthreadExtras)
+
+  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    add_subdirectory(StdlibUnittestFoundationExtras)
+    add_subdirectory(SwiftReflectionTest)
+  endif()
 endif()