fix https://github.com/icsharpcode/ILSpy/issues/2288

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -515,15 +515,16 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						Arguments = { left(), right() }
 					}, method.ReturnType);
 				case 4:
-					if (!invocation.Arguments[2].MatchLdcI4(out var isLifted))
+					if (!invocation.Arguments[2].MatchLdcI4(out var isLiftedToNull))
 						return (null, SpecialType.UnknownType);
 					if (!MatchGetMethodFromHandle(invocation.Arguments[3], out method))
 						return (null, SpecialType.UnknownType);
-					if (isLifted != 0)
+					bool isLifted = NullableType.IsNullable(leftType);
+					if (isLifted)
 						method = CSharpOperators.LiftUserDefinedOperator((IMethod)method);
 					return (() => new Call((IMethod)method) {
 						Arguments = { left(), right() }
-					}, method.ReturnType);
+					}, isLiftedToNull != 0 ? NullableType.Create(method.Compilation, method.ReturnType) : method.ReturnType);
 				default:
 					return (null, SpecialType.UnknownType);
 			}
@@ -729,13 +730,12 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			var (right, rightType) = ConvertInstruction(invocation.Arguments[1]);
 			if (right == null)
 				return (null, SpecialType.UnknownType);
-			if (invocation.Arguments.Count == 4 && invocation.Arguments[2].MatchLdcI4(out var isLifted) && MatchGetMethodFromHandle(invocation.Arguments[3], out var method))
+			if (invocation.Arguments.Count == 4 && invocation.Arguments[2].MatchLdcI4(out var isLiftedToNull) && MatchGetMethodFromHandle(invocation.Arguments[3], out var method))
 			{
-				if (isLifted != 0)
-				{
+				bool isLifted = NullableType.IsNullable(leftType);
+				if (isLifted)
 					method = CSharpOperators.LiftUserDefinedOperator((IMethod)method);
-				}
-				return (() => new Call((IMethod)method) { Arguments = { left(), right() } }, method.ReturnType);
+				return (() => new Call((IMethod)method) { Arguments = { left(), right() } }, isLiftedToNull != 0 ? NullableType.Create(method.Compilation, method.ReturnType) : method.ReturnType);
 			}
 			var rr = resolver.ResolveBinaryOperator(kind.ToBinaryOperatorType(), new ResolveResult(leftType), new ResolveResult(rightType)) as OperatorResolveResult;
 			if (rr != null && !rr.IsError && rr.UserDefinedOperatorMethod != null)
@@ -979,15 +979,16 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						Arguments = { left(), right() }
 					}, method.ReturnType);
 				case 4:
-					if (!invocation.Arguments[2].MatchLdcI4(out var isLifted))
+					if (!invocation.Arguments[2].MatchLdcI4(out var isLiftedToNull))
 						return (null, SpecialType.UnknownType);
 					if (!MatchGetMethodFromHandle(invocation.Arguments[3], out method))
 						return (null, SpecialType.UnknownType);
-					if (isLifted != 0)
+					bool isLifted = NullableType.IsNullable(leftType);
+					if (isLifted)
 						method = CSharpOperators.LiftUserDefinedOperator((IMethod)method);
 					return (() => new Call((IMethod)method) {
 						Arguments = { left(), right() }
-					}, method.ReturnType);
+					}, isLiftedToNull != 0 ? NullableType.Create(method.Compilation, method.ReturnType) : method.ReturnType);
 				default:
 					return (null, SpecialType.UnknownType);
 			}