[runtime] Fix Empty generic enumerator equality

On the CLR, two enumerators containing zero elements will
have reference equality if they are inflated with the same
generic type.

If comparing two enumerators which are not inflated (Array.Empty, for
instance), they are not equal when they contain zero elements.

This reference equality behavior is not compatible with treating
the enumerators as structs, as value types will not have reference
equality.

2 files changed, 50 insertions, 1 deletions

diff --git a/mcs/class/corlib/System/Array.cs b/mcs/class/corlib/System/Array.cs
index d699446f2b5..91424bc6f8f 100644
--- a/mcs/class/corlib/System/Array.cs
+++ b/mcs/class/corlib/System/Array.cs
@@ -68,7 +68,10 @@ namespace System
 
 		internal IEnumerator<T> InternalArray__IEnumerable_GetEnumerator<T> ()
 		{
-			return new InternalEnumerator<T> (this);
+			if (Length == 0)
+				return EmptyInternalEnumerator<T>.Value;
+			else
+				return new InternalEnumerator<T> (this);
 		}
 
 		internal void InternalArray__ICollection_Clear ()
@@ -271,6 +274,38 @@ namespace System
 			}
 		}
 
+		internal class EmptyInternalEnumerator<T> : IEnumerator<T>
+		{
+			public static readonly EmptyInternalEnumerator<T> Value = new EmptyInternalEnumerator<T> ();
+
+			public void Dispose ()
+			{
+				return;
+			}
+
+			public bool MoveNext ()
+			{
+				return false;
+			}
+
+			public T Current {
+				get {
+					throw new InvalidOperationException ("Enumeration has not started. Call MoveNext");
+				}
+			}
+
+			object IEnumerator.Current {
+				get {
+					return Current;
+				}
+			}
+
+			void IEnumerator.Reset ()
+			{
+				return;
+			}
+		}
+
 		// InternalCall Methods
 		[MethodImplAttribute (MethodImplOptions.InternalCall)]
 		extern int GetRank ();
diff --git a/mcs/class/corlib/Test/System/ArrayTest.cs b/mcs/class/corlib/Test/System/ArrayTest.cs
index 0a04041aea3..36db33d6352 100644
--- a/mcs/class/corlib/Test/System/ArrayTest.cs
+++ b/mcs/class/corlib/Test/System/ArrayTest.cs
@@ -3693,6 +3693,20 @@ public class ArrayTest
 		Assert.AreEqual (3, c.Counter);		
 	}
 
+	[Test]
+	public void EnumeratorsEquality ()
+	{
+		int [] normalBase = new int [0];
+		IEnumerable<int> specialBase = new int [0];
+
+		var firstSpecial = specialBase.GetEnumerator ();
+		var secondSpecial = specialBase.GetEnumerator ();
+		var firstNormal = normalBase.GetEnumerator ();
+		var secondNormal = normalBase.GetEnumerator ();
+
+		Assert.IsFalse (object.ReferenceEquals (firstNormal, secondNormal));
+		Assert.IsTrue (object.ReferenceEquals (firstSpecial, secondSpecial));
+	}
 
 	[Test]
 	public void JaggedArrayCtor ()