From a208b628e967d943f52b0af06981093910d23905 Mon Sep 17 00:00:00 2001
From: Marek Safar <marek.safar@gmail.com>
Date: Fri, 16 Aug 2013 15:18:49 +0200
Subject: [PATCH] Implements lifted nullable comparison with null

---
 .../BinaryExpression.cs                       | 44 +++++++++++++++++--
 .../ConstantExpression.cs                     |  6 +++
 .../System.Linq.Expressions/Expression.cs     | 16 +++++--
 .../ExpressionTest_Equal.cs                   | 18 ++++++++
 4 files changed, 77 insertions(+), 7 deletions(-)

diff --git a/mcs/class/System.Core/System.Linq.Expressions/BinaryExpression.cs b/mcs/class/System.Core/System.Linq.Expressions/BinaryExpression.cs
index 03f5f5ee492..b0528917b2c 100644
--- a/mcs/class/System.Core/System.Linq.Expressions/BinaryExpression.cs
+++ b/mcs/class/System.Core/System.Linq.Expressions/BinaryExpression.cs
@@ -570,12 +570,48 @@ namespace System.Linq.Expressions {
 
 		void EmitRelationalBinary (EmitContext ec)
 		{
-			if (!IsLifted)
+			if (!IsLifted) {
 				EmitNonLiftedBinary (ec);
-			else if (IsLiftedToNull)
+				return;
+			}
+
+			if (IsLiftedToNull) {
 				EmitLiftedToNullBinary (ec);
-			else
-				EmitLiftedRelationalBinary (ec);
+				return;
+			}
+
+			if (ConstantExpression.IsNull (right) && !ConstantExpression.IsNull (left) && left.Type.IsNullable ()) {
+				EmitNullEquality (ec, left);
+				return;
+			}
+
+			if (ConstantExpression.IsNull (left) && !ConstantExpression.IsNull (right) && right.Type.IsNullable ()) {
+				EmitNullEquality (ec, right);
+				return;
+			}
+
+			EmitLiftedRelationalBinary (ec);
+		}
+
+		void EmitNullEquality (EmitContext ec, Expression e)
+		{
+			var ig = ec.ig;
+
+			if (IsLiftedToNull) {
+				e.Emit (ec);
+				if (e.Type != typeof (void))
+					ig.Emit (OpCodes.Pop);
+
+				ec.EmitNullableNew (typeof (bool?));
+				return;
+			}
+
+			var se = ec.EmitStored (e);
+			ec.EmitNullableHasValue (se);
+			if (NodeType == ExpressionType.Equal) {
+				ig.Emit (OpCodes.Ldc_I4_0);
+				ig.Emit (OpCodes.Ceq);
+			}		
 		}
 
 		void EmitLiftedUserDefinedOperator (EmitContext ec)
diff --git a/mcs/class/System.Core/System.Linq.Expressions/ConstantExpression.cs b/mcs/class/System.Core/System.Linq.Expressions/ConstantExpression.cs
index 10dd69ce8f6..2ff30c8b81c 100644
--- a/mcs/class/System.Core/System.Linq.Expressions/ConstantExpression.cs
+++ b/mcs/class/System.Core/System.Linq.Expressions/ConstantExpression.cs
@@ -51,6 +51,12 @@ namespace System.Linq.Expressions {
 		{
 			this.value = value;
 		}
+		
+		internal static bool IsNull (Expression e)
+		{
+			var c = e as ConstantExpression;
+			return c != null && c.value == null;
+		}
 
 #if !FULL_AOT_RUNTIME
 		internal override void Emit (EmitContext ec)
diff --git a/mcs/class/System.Core/System.Linq.Expressions/Expression.cs b/mcs/class/System.Core/System.Linq.Expressions/Expression.cs
index f79156284c2..696da9ff91c 100644
--- a/mcs/class/System.Core/System.Linq.Expressions/Expression.cs
+++ b/mcs/class/System.Core/System.Linq.Expressions/Expression.cs
@@ -272,6 +272,12 @@ namespace System.Linq.Expressions {
 
 					if (ltype == rtype && ultype == typeof (bool))
 						return null;
+
+					if (ltype.IsNullable () && ConstantExpression.IsNull (right) && !ConstantExpression.IsNull (left))
+						return null;
+
+					if (rtype.IsNullable () && ConstantExpression.IsNull (left) && !ConstantExpression.IsNull (right))
+						return null;
 				}
 
 				if (oper_name == "op_LeftShift" || oper_name == "op_RightShift") {
@@ -392,12 +398,16 @@ namespace System.Linq.Expressions {
 				if (!left.Type.IsNullable () && !right.Type.IsNullable ()) {
 					is_lifted = false;
 					liftToNull = false;
-					type = typeof (bool);
+					type = typeof(bool);
 				} else if (left.Type.IsNullable () && right.Type.IsNullable ()) {
 					is_lifted = true;
-					type = liftToNull ? typeof (bool?) : typeof (bool);
-				} else
+					type = liftToNull ? typeof(bool?) : typeof(bool);
+				} else if (ConstantExpression.IsNull (left) || ConstantExpression.IsNull (right)) {
+					is_lifted = true;
+					type = typeof (bool);
+				} else {			
 					throw new InvalidOperationException ();
+				}
 			} else {
 				var parameters = method.GetParameters ();
 
diff --git a/mcs/class/System.Core/Test/System.Linq.Expressions/ExpressionTest_Equal.cs b/mcs/class/System.Core/Test/System.Linq.Expressions/ExpressionTest_Equal.cs
index 06612bdfa18..749d78e40f8 100644
--- a/mcs/class/System.Core/Test/System.Linq.Expressions/ExpressionTest_Equal.cs
+++ b/mcs/class/System.Core/Test/System.Linq.Expressions/ExpressionTest_Equal.cs
@@ -458,5 +458,23 @@ namespace MonoTests.System.Linq.Expressions
 			Assert.AreEqual (false, eq (Foo.Bar, null));
 			Assert.AreEqual (true, eq (null, null));
 		}
+
+		[Test]
+		public void NullableNullEqual ()
+		{
+			var param = Expression.Parameter (typeof (DateTime?), "x");
+
+			var node = Expression.Equal (param, Expression.Constant (null));
+
+			Assert.IsTrue (node.IsLifted);
+			Assert.IsFalse (node.IsLiftedToNull);
+			Assert.AreEqual (typeof (bool), node.Type);
+			Assert.IsNull (node.Method);
+
+			var eq = Expression.Lambda<Func<DateTime?, bool>> (node, new [] { param }).Compile ();
+
+			Assert.AreEqual (true, eq (null));
+			Assert.AreEqual (false, eq (DateTime.Now));
+		}
 	}
 }
-- 
2.25.1