CSHARP-4316: Fixed translation of types similar to Nullable<T>

Author: Vladimir

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MemberExpressionToAggregationExpressionTranslator.cs

@@ -26,6 +26,7 @@
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 using MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.PropertyTranslators;
+using MongoDB.Driver.Support;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators
 {
@@ -36,7 +37,7 @@ public static AggregationExpression Translate(TranslationContext context, Member
             var containerExpression = expression.Expression;
             var member = expression.Member;
 
-            if (member is PropertyInfo property)
+            if (member is PropertyInfo property && property.DeclaringType.IsNullable())
             {
                 switch (property.Name)
                 {

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4316Tests.cs

@@ -0,0 +1,60 @@
+using System.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4316Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Nullable_type_translate_should_work()
+        {
+            var collection = GetCollection<ModelContainNullableType>();
+            var fluent = collection.Aggregate()
+                .Group(x => new { Some = x.Some.Value, x.AnotherKeyGroup, x.Some.HasValue }, x => x.Select(t => t));
+
+            var stages = Translate(collection, fluent);
+            var expected = new []
+            {
+                "{ $group : {'_id' : { Some : '$Some', AnotherKeyGroup : '$AnotherKeyGroup', HasValue : { $ne : ['$Some', null] }}, __agg0 : { '$push' : '$$ROOT'}}}",
+                "{ $project : { '_v' : '$__agg0', _id : 0 } }"
+            };
+
+            AssertStages(stages, expected);
+        }
+
+        [Fact]
+        public void Type_contains_property_value_translate_should_work()
+        {
+            var collection = GetCollection<ModelWithoutNullable>();
+            var fluent = collection.Aggregate()
+                .Group(x => new { Key1 = x.Property.Value, Key2 = x.SomeData, Key3 = x.Property.HasValue}, x => x.Select(t => t));
+
+            var stages = Translate(collection, fluent);
+            var expected = new []
+            {
+                "{ $group : {'_id' : { Key1 : '$Property.Value', Key2 : '$SomeData', Key3 : '$Property.HasValue' }, __agg0 : { '$push' : '$$ROOT'}}}",
+                "{ $project : { '_v' : '$__agg0', _id : 0 } }"
+            };
+            AssertStages(stages, expected);
+        }
+
+        public class ModelContainNullableType
+        {
+            public int? Some { get; set; }
+            public float AnotherKeyGroup { get; set; }
+
+        }
+
+        public class ModelWithoutNullable
+        {
+            public CustomTypeLikeNullable Property { get; set; }
+            public int SomeData { get; set; }
+        }
+
+        public class CustomTypeLikeNullable
+        {
+            public string Value { get; set; }
+            public bool HasValue { get; set; }
+        }
+    }
+}