Index: include/utility
===================================================================
--- include/utility
+++ include/utility
@@ -924,6 +924,12 @@
 template <class _Tp>
 using __is_inplace_type = __is_inplace_type_imp<__uncvref_t<_Tp>>;
 
+template <class _Tp> struct __is_inplace_index_imp : false_type {};
+template <size_t _Idx> struct __is_inplace_index_imp<in_place_index_t<_Idx>> : true_type {};
+
+template <class _Tp>
+using __is_inplace_index = __is_inplace_index_imp<_Tp>;
+
 #endif // _LIBCPP_STD_VER > 14
 
 template <class _Arg, class _Result>