Imported Upstream version 6.4.0.137

Former-commit-id: 943baa9f16a098c33e129777827f3a9d20da00d6

mcs/class/System.Private.CoreLib/System.Collections.Generic/Comparer.cs

@@ -0,0 +1,46 @@
+using System.Runtime.CompilerServices;
+
+namespace System.Collections.Generic
+{
+	partial class Comparer<T>
+	{
+        static volatile Comparer<T> defaultComparer;
+
+		public static Comparer<T> Default {
+			get {
+                Comparer<T> comparer = defaultComparer;
+                if (comparer == null) {
+                    comparer = CreateComparer();
+                    defaultComparer = comparer;
+                }
+                return comparer;
+			}
+		}
+
+        static Comparer<T> CreateComparer() {
+            RuntimeType t = (RuntimeType)typeof(T);
+
+                if (typeof(IComparable<T>).IsAssignableFrom(t))
+                    return (Comparer<T>)RuntimeType.CreateInstanceForAnotherGenericParameter (typeof(GenericComparer<>), t);
+
+				// If T is a Nullable<U> where U implements IComparable<U> return a NullableComparer<U>
+				if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>)) {
+					RuntimeType u = (RuntimeType)t.GetGenericArguments()[0];
+					if (typeof(IComparable<>).MakeGenericType (u).IsAssignableFrom (u))
+						return (Comparer<T>)RuntimeType.CreateInstanceForAnotherGenericParameter (typeof(NullableComparer<>), u);
+				}
+
+				if (t.IsEnum)
+					return (Comparer<T>)RuntimeType.CreateInstanceForAnotherGenericParameter (typeof(EnumComparer<>), t);
+
+				// Otherwise return an ObjectComparer<T>
+				return new ObjectComparer<T> ();
+		}
+	}
+
+	partial class EnumComparer<T>
+	{
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
+		public override int Compare (T x, T y) => JitHelpers.EnumCompareTo (x, y);
+	}
+}