Merge pull request #27962 from mikeash/isbackdeploying-fix

Author: swift-ci

include/swift/Runtime/Config.h

@@ -160,6 +160,7 @@
 # else
 #  define SWIFT_CLASS_IS_SWIFT_MASK _swift_classIsSwiftMask
 #  define SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE 1
+#  define SWIFT_BUILD_HAS_BACK_DEPLOYMENT 1
 #  include "BackDeployment.h"
 
 # endif

stdlib/public/runtime/SwiftObject.mm

@@ -201,7 +201,10 @@ + (void)initialize {
   // runs on older OSes in certain testing scenarios, so that doesn't matter.
   // Only perform the check on newer OSes where the value should definitely
   // match.
-  if (!_swift_isBackDeploying()) {
+#  if SWIFT_BUILD_HAS_BACK_DEPLOYMENT
+  if (!_swift_isBackDeploying())
+#  endif
+  {
     assert(&objc_debug_isa_class_mask);
     assert(objc_debug_isa_class_mask == SWIFT_ISA_MASK);
   }