Fixed resolving equality and relational operators on nullable types.

ICSharpCode.NRefactory.CSharp/Resolver/CSharpResolver.cs

@@ -692,6 +692,16 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 						} else if (lhsType is PointerType && rhsType is PointerType) {
 							return BinaryOperatorResolveResult(compilation.FindType(KnownTypeCode.Boolean), lhs, op, rhs);
 						}
+						if (op == BinaryOperatorType.Equality || op == BinaryOperatorType.InEquality) {
+							if (lhsType.Kind == TypeKind.Null && NullableType.IsNullable(rhs.Type)
+							    || rhsType.Kind == TypeKind.Null && NullableType.IsNullable(lhs.Type))
+							{
+								// §7.10.9 Equality operators and null
+								// "x == null", "null == x", "x != null" and "null != x" are valid
+								// even if the struct does not define operator ==.
+								return BinaryOperatorResolveResult(compilation.FindType(KnownTypeCode.Boolean), lhs, op, rhs);
+							}
+						}
 						switch (op) {
 							case BinaryOperatorType.Equality:
 								methodGroup = operators.EqualityOperators;
@@ -900,10 +910,10 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 			TypeCode lhsCode = ReflectionHelper.GetTypeCode(NullableType.GetUnderlyingType(lhs.Type));
 			TypeCode rhsCode = ReflectionHelper.GetTypeCode(NullableType.GetUnderlyingType(rhs.Type));
 			// if one of the inputs is the null literal, promote that to the type of the other operand
-			if (isNullable && SpecialType.NullType.Equals(lhs.Type)) {
+			if (isNullable && SpecialType.NullType.Equals(lhs.Type) && rhsCode >= TypeCode.Boolean && rhsCode <= TypeCode.Decimal) {
 				lhs = CastTo(rhsCode, isNullable, lhs, allowNullableConstants);
 				lhsCode = rhsCode;
-			} else if (isNullable && SpecialType.NullType.Equals(rhs.Type)) {
+			} else if (isNullable && SpecialType.NullType.Equals(rhs.Type) && lhsCode >= TypeCode.Boolean && lhsCode <= TypeCode.Decimal) {
 				rhs = CastTo(lhsCode, isNullable, rhs, allowNullableConstants);
 				rhsCode = lhsCode;
 			}
@@ -1075,9 +1085,13 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 		
 		LiftedUserDefinedOperator LiftUserDefinedOperator(IMethod m)
 		{
-			IType returnType = m.ReturnType;
+			if (IsComparisonOperator(m)) {
-			if (!NullableType.IsNonNullableValueType(returnType))
+				if (!m.ReturnType.Equals(compilation.FindType(KnownTypeCode.Boolean)))
 					return null; // cannot lift this operator
+			} else {
+				if (!NullableType.IsNonNullableValueType(m.ReturnType))
+					return null; // cannot lift this operator
+			}
 			for (int i = 0; i < m.Parameters.Count; i++) {
 				if (!NullableType.IsNonNullableValueType(m.Parameters[i].Type))
 					return null; // cannot lift this operator
@@ -1085,6 +1099,23 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 			return new LiftedUserDefinedOperator(m);
 		}
 		
+		static bool IsComparisonOperator(IMethod m)
+		{
+			var type = OperatorDeclaration.GetOperatorType(m.Name);
+			if (type.HasValue) {
+				switch (type.Value) {
+					case OperatorType.Equality:
+					case OperatorType.Inequality:
+					case OperatorType.GreaterThan:
+					case OperatorType.LessThan:
+					case OperatorType.GreaterThanOrEqual:
+					case OperatorType.LessThanOrEqual:
+						return true;
+				}
+			}
+			return false;
+		}
+		
 		sealed class LiftedUserDefinedOperator : SpecializedMethod, OverloadResolution.ILiftedOperator
 		{
 			internal readonly IParameterizedMember nonLiftedOperator;
@@ -1094,6 +1125,9 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 				       EmptyList<IType>.Instance, new MakeNullableVisitor(nonLiftedMethod.Compilation))
 			{
 				this.nonLiftedOperator = nonLiftedMethod;
+				// Comparison operators keep the 'bool' return type even when lifted.
+				if (IsComparisonOperator(nonLiftedMethod))
+					this.returnType = nonLiftedMethod.ReturnType;
 			}
 			
 			public IList<IParameter> NonLiftedParameters {

ICSharpCode.NRefactory.Tests/CSharp/Resolver/BinaryOperatorTests.cs

@@ -342,6 +342,9 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 			
 			AssertType(typeof(bool), resolver.ResolveBinaryOperator(
 				BinaryOperatorType.InEquality, MakeResult(typeof(int*)), MakeResult(typeof(uint*))));
+			
+			AssertType(typeof(bool), resolver.ResolveBinaryOperator(
+				BinaryOperatorType.InEquality, MakeResult(typeof(bool?)), MakeConstant(null)));
 		}
 		
 		[Test]
@@ -579,5 +582,37 @@ class Test {
 			Assert.IsNull(irr.UserDefinedOperatorMethod);
 			Assert.AreEqual("System.Byte", irr.Type.ReflectionName);
 		}
+		
+		[Test]
+		public void CompareNullableStructWithNullLiteral()
+		{
+			string program = @"
+struct X { }
+class Test {
+	static void Inc(X? x) {
+		var c = $x == null$;
+	}
+}";
+			var irr = Resolve<OperatorResolveResult>(program);
+			Assert.IsFalse(irr.IsError);
+			Assert.AreEqual(compilation.FindType(KnownTypeCode.Boolean), irr.Type);
+		}
+		
+		[Test]
+		public void LiftedEqualityOperator()
+		{
+			string program = @"
+struct X {
+	public static bool operator ==(X a, X b) {}
+}
+class Test {
+	static void Inc(X? x) {
+		var c = $x == x$;
+	}
+}";
+			var irr = Resolve<OperatorResolveResult>(program);
+			Assert.IsFalse(irr.IsError);
+			Assert.AreEqual(compilation.FindType(KnownTypeCode.Boolean), irr.Type);
+		}
 	}
 }

ICSharpCode.NRefactory/TypeSystem/Implementation/SpecializedMember.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 	{
 		readonly IType declaringType;
 		readonly IMember memberDefinition;
-		IType returnType;
+		protected IType returnType;
 		
 		protected SpecializedMember(IType declaringType, IMember memberDefinition)
 		{