diff --git a/include/boost/config/compiler/clang.hpp b/include/boost/config/compiler/clang.hpp
index 3ba62096..0e07b744 100644
--- a/include/boost/config/compiler/clang.hpp
+++ b/include/boost/config/compiler/clang.hpp
@@ -39,9 +39,11 @@
 #  define BOOST_NO_TYPEID
 #endif
 
-#if defined(__is_identifier) && !__is_identifier(__int64) && !defined(__GNUC__)
+#ifdef __is_identifier
+#if !__is_identifier(__int64) && !defined(__GNUC__)
 #  define BOOST_HAS_MS_INT64
 #endif
+#endif
 
 #if __has_include(<stdint.h>)
 #  define BOOST_HAS_STDINT_H