Index: include/atomic
===================================================================
--- include/atomic
+++ include/atomic
@@ -584,9 +584,18 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// Figure out what the underlying type for `memory_order` would be if it were
+// declared as an unscoped enum (accounting for -fshort-enums). Use this result
+// to pin the underlying type in C++20.
+struct __legacy_memory_order {
+  enum __enum_type { __v1, __v2, __v3, __v4, __v5, __v6 };
+};
+
+typedef underlying_type<__legacy_memory_order::__enum_type>::type __memory_order_underlying_t;
+
 #if _LIBCPP_STD_VER > 17
 
-enum class memory_order {
+enum class memory_order : __memory_order_underlying_t {
   relaxed, consume, acquire, release, acq_rel, seq_cst
 };
 
@@ -606,8 +615,8 @@
 
 #endif // _LIBCPP_STD_VER > 17
 
-typedef underlying_type<memory_order>::type __memory_order_underlying_t;
-
+static_assert(is_same<underlying_type<memory_order>::type, __memory_order_underlying_t>::value,
+  "unexpected underlying type for std::memory_order");
 
 #if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP) || \
 	defined(_LIBCPP_ATOMIC_ONLY_USE_BUILTINS)