Add enums to Enumerable.Order implicitly stable check (#99533)

stephentoub · web-flow · commit 28978b0662e7 · 2024-03-11T21:52:02.000-04:00
* Add enums to Enumerable.Order implicitly stable check

* Address PR feedback
diff --git a/src/libraries/System.Linq/src/System/Linq/OrderBy.cs b/src/libraries/System.Linq/src/System/Linq/OrderBy.cs
@@ -140,14 +140,27 @@ public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(this I
 
         /// <summary>Gets whether the results of an unstable sort will be observably the same as a stable sort.</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        internal static bool TypeIsImplicitlyStable<T>() =>
-            typeof(T) == typeof(sbyte) || typeof(T) == typeof(byte) ||
-            typeof(T) == typeof(int) || typeof(T) == typeof(uint) ||
-            typeof(T) == typeof(short) || typeof(T) == typeof(ushort) ||
-            typeof(T) == typeof(long) || typeof(T) == typeof(ulong) ||
-            typeof(T) == typeof(Int128) || typeof(T) == typeof(UInt128) ||
-            typeof(T) == typeof(nint) || typeof(T) == typeof(nuint) ||
-            typeof(T) == typeof(bool) || typeof(T) == typeof(char);
+        internal static bool TypeIsImplicitlyStable<T>()
+        {
+            Type t = typeof(T);
+            if (typeof(T).IsEnum)
+            {
+                t = typeof(T).GetEnumUnderlyingType();
+            }
+
+            // Check for integral primitive types that compare equally iff they have the same bit pattern.
+            // bool is included because, even though technically it can have 256 different values, anything
+            // other than 0/1 is only producible using unsafe code. It's tempting to include a type like string
+            // here, as it's so commonly used with ordering, but two different string objects can compare equally,
+            // and their reference identity can be observable in a stable vs unstable sort.
+            return
+                t == typeof(sbyte) || t == typeof(byte) || t == typeof(bool) ||
+                t == typeof(short) || t == typeof(ushort) || t == typeof(char) ||
+                t == typeof(int) || t == typeof(uint) ||
+                t == typeof(long) || t == typeof(ulong) ||
+                t == typeof(Int128) || t == typeof(UInt128) ||
+                t == typeof(nint) || t == typeof(nuint);
+        }
     }
 
     public interface IOrderedEnumerable<out TElement> : IEnumerable<TElement>
