NH-3850 - Polymorphic Linq query results aggregators failures (nhibernate#584)

* NH-3850 - Test cases for polymorphic Linq query results aggregators failures

* NH-3850 - Fix for polymorphic Linq query results aggregators failures.

* NH-3850 - Amending test cases for polymorphic Linq query results aggregators failures: acknowledging cases which will not be fixed.

Author: fredericDelaporte

src/NHibernate.Test/Linq/ByMethod/SumTests.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using NUnit.Framework;
 
 namespace NHibernate.Test.Linq.ByMethod
@@ -14,7 +15,10 @@ public void EmptySumDecimal()
 				{
 					db.OrderLines.Where(ol => false).Sum(ol => ol.Discount);
 				},
-				Throws.InstanceOf<HibernateException>());
+				// Before NH-3850
+				Throws.InstanceOf<HibernateException>()
+				// After NH-3850
+				.Or.InstanceOf<InvalidOperationException>());
 		}
 
 		[Test]

src/NHibernate.Test/NHSpecificTest/Dates/FixtureBase.cs

@@ -32,10 +32,6 @@ protected override bool AppliesTo(Dialect.Dialect dialect)
 			{
 				return false;
 			}
-			catch (Exception)
-			{
-				Assert.Fail("Probably a bug in the test case.");
-			}
 
 			return true;
 		}

src/NHibernate.Test/NHSpecificTest/NH3850/Domain.cs

@@ -0,0 +1,49 @@
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.NH3850
+{
+	public abstract class DomainClassBase
+	{
+		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual int? Integer { get; set; }
+		public virtual long? Long { get; set; }
+		public virtual decimal? Decimal { get; set; }
+		public virtual double? Double { get; set; }
+		public virtual DateTime? DateTime { get; set; }
+		public virtual DateTimeOffset? DateTimeOffset { get; set; }
+		public virtual decimal NonNullableDecimal { get; set; }
+	}
+
+	public class DomainClassAExtendingB : DomainClassBExtendedByA
+	{
+	}
+
+	public class DomainClassBExtendedByA : DomainClassBase
+	{
+	}
+
+	public class DomainClassCExtendedByD : DomainClassBase
+	{
+	}
+
+	public class DomainClassDExtendingC : DomainClassCExtendedByD
+	{
+	}
+
+	public class DomainClassE : DomainClassBase
+	{
+	}
+
+	public class DomainClassF : DomainClassBase
+	{
+	}
+
+	public class DomainClassGExtendedByH : DomainClassBase
+	{
+	}
+
+	public class DomainClassHExtendingG : DomainClassGExtendedByH
+	{
+	}
+}