Add Enum Equals support

Author: Tomas Lukac

src/NHibernate.Test/Linq/FunctionTests.cs

@@ -491,6 +491,16 @@ where item.Discount.Equals(-1)
 			ObjectDumper.Write(query);
 		}
 
+		[Test]
+		public void WhereEnumEqual()
+		{
+			var query = from item in db.PatientRecords
+						where item.Gender.Equals(Gender.Female)
+						select item;
+
+			ObjectDumper.Write(query);
+		}
+
 		[Test]
 		public void WhereEquatableEqual()
 		{

src/NHibernate/Linq/Functions/DefaultLinqToHqlGeneratorsRegistry.cs

@@ -39,6 +39,7 @@ public DefaultLinqToHqlGeneratorsRegistry()
 			this.Merge(new EndsWithGenerator());
 			this.Merge(new ContainsGenerator());
 			this.Merge(new EqualsGenerator());
+			this.Merge(new EnumEqualsGenerator());
 			this.Merge(new ToUpperGenerator());
 			this.Merge(new ToLowerGenerator());
 			this.Merge(new SubStringGenerator());

src/NHibernate/Linq/Functions/EnumEqualsGenerator.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq.Expressions;
+using System.Reflection;
+using NHibernate.Hql.Ast;
+using NHibernate.Linq.Visitors;
+using NHibernate.Util;
+
+namespace NHibernate.Linq.Functions
+{
+	/// <summary>
+	/// Not using <see cref="EqualsGenerator"/> class as there is very special handling of enums in expressions and equality operator
+	/// </summary>
+	public class EnumEqualsGenerator : BaseHqlGeneratorForMethod
+	{
+		internal static HashSet<MethodInfo> Methods = new HashSet<MethodInfo>
+		{
+			ReflectHelper.GetMethodDefinition<Enum>(x => x.Equals(default(object))),
+			ReflectHelper.GetMethodDefinition<IEquatable<Enum>>(x => x.Equals(default(Enum)))
+		};
+
+		public EnumEqualsGenerator()
+		{
+			SupportedMethods = Methods;
+		}
+
+		public override bool AllowsNullableReturnType(MethodInfo method) => false;
+
+		public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject, ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
+		{
+			Expression lhs = arguments.Count == 1 ? targetObject : arguments[0];
+			Expression rhs = arguments.Count == 1 ? arguments[0] : arguments[1];
+
+			return treeBuilder.Equality(visitor.Visit(lhs).AsExpression(), visitor.Visit(rhs).AsExpression());
+		}
+	}
+}