diff --git a/src/ios.cpp b/src/ios.cpp
index 9d9daa9..23e3ee0 100644
--- a/src/ios.cpp
+++ b/src/ios.cpp
@@ -152,7 +152,7 @@
 }
 
 // xalloc
-#if __has_feature(cxx_atomic) && !defined(_LIBCPP_HAS_NO_THREADS)
+#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
 atomic<int> ios_base::__xindex_ = ATOMIC_VAR_INIT(0);
 #else
 int ios_base::__xindex_ = 0;