CSHARP-1468: Fix conversion issue to base class.

Author: craiggwilson

src/MongoDB.Driver.Tests/FilterDefinitionBuilderTests.cs

@@ -824,6 +824,19 @@ public void Type_Typed()
             Assert(subject.Type("FirstName", BsonType.String), "{fn: {$type: 2}}");
         }
 
+        [Test]
+        public void Generic_type_constraint_causing_base_class_conversion()
+        {
+            var filter = TypeConstrainedFilter<Twin>(21);
+
+            Assert(filter, "{ age: 21 }");
+        }
+
+        private FilterDefinition<T> TypeConstrainedFilter<T>(int age) where T : Person
+        {
+            return CreateSubject<T>().Eq(x => x.Age, age);
+        }
+
         private void Assert<TDocument>(FilterDefinition<TDocument> filter, string expected)
         {
             Assert(filter, BsonDocument.Parse(expected));

src/MongoDB.Driver/Linq/Processors/SerializationBinder.cs

@@ -161,6 +161,20 @@ protected override Expression VisitMethodCall(MethodCallExpression node)
             return base.VisitMethodCall(node);
         }
 
+        protected override Expression VisitUnary(UnaryExpression node)
+        {
+            var newNode = (UnaryExpression)base.VisitUnary(node);
+            if (newNode.NodeType == ExpressionType.Convert || newNode.NodeType == ExpressionType.ConvertChecked)
+            {
+                if (newNode.Method == null && !newNode.IsLiftedToNull && newNode.Type.IsAssignableFrom(newNode.Operand.Type))
+                {
+                    return newNode.Operand;
+                }
+            }
+
+            return newNode;
+        }
+
         // private methods
         private Expression BindElementAt(MethodCallExpression node)
         {