Fix nullable entity comparison with null

src/NHibernate.Test/Async/Hql/EntityJoinHqlTest.cs

@@ -342,8 +342,12 @@ public async Task NullableEntityProjectionAsync()
 
 				var fullList = await (session.Query<NullableOwner>().Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToListAsync());
 				var withValidManyToOneList = await (session.Query<NullableOwner>().Where(x => x.ManyToOne != null).Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToListAsync());
+				var withValidManyToOneList2 = await (session.CreateQuery("from NullableOwner ex where not ex.ManyToOne is null").ListAsync<NullableOwner>());
+				var withNullManyToOneList = await (session.Query<NullableOwner>().Where(x => x.ManyToOne == null).ToListAsync());
 				Assert.That(fullList.Count, Is.EqualTo(2));
 				Assert.That(withValidManyToOneList.Count, Is.EqualTo(0));
+				Assert.That(withValidManyToOneList2.Count, Is.EqualTo(0));
+				Assert.That(withNullManyToOneList.Count, Is.EqualTo(2));
 			}
 		}
 

src/NHibernate.Test/Hql/EntityJoinHqlTest.cs

@@ -330,8 +330,12 @@ public void NullableEntityProjection()
 
 				var fullList = session.Query<NullableOwner>().Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToList();
 				var withValidManyToOneList = session.Query<NullableOwner>().Where(x => x.ManyToOne != null).Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToList();
+				var withValidManyToOneList2 = session.CreateQuery("from NullableOwner ex where not ex.ManyToOne is null").List<NullableOwner>();
+				var withNullManyToOneList = session.Query<NullableOwner>().Where(x => x.ManyToOne == null).ToList();
 				Assert.That(fullList.Count, Is.EqualTo(2));
 				Assert.That(withValidManyToOneList.Count, Is.EqualTo(0));
+				Assert.That(withValidManyToOneList2.Count, Is.EqualTo(0));
+				Assert.That(withNullManyToOneList.Count, Is.EqualTo(2));
 			}
 		}
 

src/NHibernate/Hql/Ast/ANTLR/HqlSqlWalker.cs

@@ -42,6 +42,7 @@ public partial class HqlSqlWalker
 		private SelectClause _selectClause;
 		private readonly AliasGenerator _aliasGenerator = new AliasGenerator();
 		private readonly ASTPrinter _printer = new ASTPrinter();
+		private bool _isNullComparison;
 
 		//
 		//Maps each top-level result variable to its SelectExpression;
@@ -1203,6 +1204,8 @@ public IASTFactory ASTFactory
 			}
 		}
 
+		internal bool IsNullComparison => _isNullComparison;
+
 		public void AddQuerySpaces(string[] spaces)
 		{
 			for (int i = 0; i < spaces.Length; i++)

src/NHibernate/Hql/Ast/ANTLR/HqlSqlWalker.g

@@ -380,7 +380,7 @@ comparisonExpr
 	| ^(NOT_BETWEEN exprOrSubquery exprOrSubquery exprOrSubquery)
 	| ^(IN exprOrSubquery inRhs )
 	| ^(NOT_IN exprOrSubquery inRhs )
-	| ^(IS_NULL exprOrSubquery)
+	| ^(IS_NULL { _isNullComparison = true; } exprOrSubquery { _isNullComparison = false; })
 	| ^(IS_NOT_NULL exprOrSubquery)
 //	| ^(IS_TRUE expr)
 //	| ^(IS_FALSE expr)

src/NHibernate/Hql/Ast/ANTLR/Tree/DotNode.cs

@@ -389,8 +389,6 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 			bool joinIsNeeded;
 
 			//For nullable entity comparisons we always need to add join (like not constrained one-to-one or not-found ignore associations)
-			//NOTE: This fix is not fully correct. It doesn't work for comparisons with null (where e.OneToOneProp is null)
-			// as by default implicit join is generated and to work propelry left join is required (see GH-2611)
 			bool comparisonWithNullableEntity = entityType.IsNullable && Walker.IsComparativeExpressionClause;
 
 			if ( IsDotNode( parent ) ) 
@@ -417,8 +415,14 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 				               || comparisonWithNullableEntity;
 			}
 
-			if ( joinIsNeeded ) 
+			if ( joinIsNeeded )
 			{
+				if (comparisonWithNullableEntity && Walker.IsNullComparison)
+				{
+					implicitJoin = false;
+					_joinType = JoinType.LeftOuterJoin;
+				}
+
 				DereferenceEntityJoin( classAlias, entityType, implicitJoin, parent );
 				if (comparisonWithNullableEntity)
 				{