diff --git a/stl/inc/atomic b/stl/inc/atomic
index 9b6d178f195..a8d5ceecb73 100644
--- a/stl/inc/atomic
+++ b/stl/inc/atomic
@@ -161,7 +161,7 @@ _Ty kill_dependency(_Ty _Arg) noexcept { // "magic" template that kills dependen
 // FUNCTION _Check_memory_order
 inline void _Check_memory_order(const memory_order _Order) noexcept {
     // check that _Order is a valid memory_order
-    if (static_cast<unsigned int>(_Order) > memory_order_seq_cst) {
+    if (static_cast<unsigned int>(_Order) > static_cast<unsigned int>(memory_order_seq_cst)) {
         _INVALID_MEMORY_ORDER;
     }
 }
@@ -230,7 +230,7 @@ _NODISCARD inline memory_order _Combine_cas_memory_orders(
 
     _Check_memory_order(_Success);
     _Check_load_memory_order(_Failure);
-    return _Combined_memory_orders[_Success][_Failure];
+    return _Combined_memory_orders[static_cast<int>(_Success)][static_cast<int>(_Failure)];
 }
 
 // FUNCTION TEMPLATE _Atomic_reinterpret_as
diff --git a/stl/inc/xatomic.h b/stl/inc/xatomic.h
index 28357d8a313..1562df01000 100644
--- a/stl/inc/xatomic.h
+++ b/stl/inc/xatomic.h
@@ -55,6 +55,16 @@ _STL_DISABLE_CLANG_WARNINGS
 
 _STD_BEGIN
 
+#if _HAS_CXX20
+// ENUM CLASS memory_order
+enum class memory_order : int { relaxed, consume, acquire, release, acq_rel, seq_cst };
+inline constexpr memory_order memory_order_relaxed = memory_order::relaxed;
+inline constexpr memory_order memory_order_consume = memory_order::consume;
+inline constexpr memory_order memory_order_acquire = memory_order::acquire;
+inline constexpr memory_order memory_order_release = memory_order::release;
+inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel;
+inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst;
+#else // _HAS_CXX20
 // ENUM memory_order
 enum memory_order {
     memory_order_relaxed,
@@ -64,6 +74,7 @@ enum memory_order {
     memory_order_acq_rel,
     memory_order_seq_cst
 };
+#endif // _HAS_CXX20
 
 using _Atomic_counter_t = unsigned long;
 
diff --git a/stl/inc/yvals_core.h b/stl/inc/yvals_core.h
index 2a40f1bb0f9..84213b67189 100644
--- a/stl/inc/yvals_core.h
+++ b/stl/inc/yvals_core.h
@@ -22,6 +22,7 @@
 // P0020R6 atomic<float>, atomic<double>, atomic<long double>
 // P0318R1 unwrap_reference, unwrap_ref_decay
 // P0325R4 to_array()
+// P0439R0 enum class memory_order
 // P0457R2 starts_with()/ends_with() For basic_string/basic_string_view
 // P0458R2 contains() For Ordered And Unordered Associative Containers
 // P0463R1 endian