Try gating the Darwin dependencies on SWIFT_ENABLE_REFLECTION

Author: ian-twilightcoder

stdlib/private/CMakeLists.txt

@@ -1,7 +1,12 @@
-if(SWIFT_BUILD_SDK_OVERLAY OR (SWIFT_BUILD_TEST_SUPPORT_MODULES AND NOT SWIFT_BUILD_DYNAMIC_SDK_OVERLAY_default))
+if(SWIFT_BUILD_SDK_OVERLAY
+   OR (SWIFT_BUILD_TEST_SUPPORT_MODULES
+       AND NOT SWIFT_BUILD_DYNAMIC_SDK_OVERLAY_default
+       AND SWIFT_ENABLE_REFLECTION))
   # SwiftPrivateThreadExtras makes use of Darwin/Glibc, which is part of the
   # SDK overlay. If the SDK overlay doesn't build by default, then it should
   # be available in the SDK and DifferentiationUnittest can still be built.
+  # However, the overlay in the SDK requires the standard library to have
+  # reflection enabled.
   if(SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING)
     add_subdirectory(DifferentiationUnittest)
   endif()
@@ -14,7 +19,10 @@ if(SWIFT_BUILD_SDK_OVERLAY OR SWIFT_BUILD_TEST_SUPPORT_MODULES)
   add_subdirectory(StdlibCollectionUnittest)
 endif()
 
-if(SWIFT_BUILD_SDK_OVERLAY OR (SWIFT_BUILD_TEST_SUPPORT_MODULES AND NOT SWIFT_BUILD_DYNAMIC_SDK_OVERLAY_default))
+if(SWIFT_BUILD_SDK_OVERLAY
+   OR (SWIFT_BUILD_TEST_SUPPORT_MODULES
+       AND NOT SWIFT_BUILD_DYNAMIC_SDK_OVERLAY_default
+       AND SWIFT_ENABLE_REFLECTION))
   add_subdirectory(SwiftPrivateLibcExtras)
   add_subdirectory(SwiftPrivateThreadExtras)