Enable Not node handling in HqlParser.NegateNode (#3390)

src/NHibernate.Test/Async/NHSpecificTest/GH3327/Fixture.cs

@@ -59,5 +59,21 @@ WHERE NOT (
 					)");
 			Assert.That((await (q.ListAsync()))[0], Is.EqualTo(0));
 		}
+
+		[Test]
+		public async Task NotNotExistsNegatedAsync()
+		{
+			using var log = new SqlLogSpy();
+			using var session = OpenSession();
+			var results = await (session.CreateQuery(
+				@"SELECT COUNT(ROOT.Id)
+					FROM Entity AS ROOT
+					WHERE NOT (
+						NOT EXISTS (FROM ChildEntity AS CHILD WHERE CHILD.Parent = ROOT)
+						AND NOT ROOT.Name = 'Parent'
+					)").ListAsync());
+			Assert.That(log.GetWholeLog(), Does.Not.Contains(" NOT ").IgnoreCase);
+			Assert.That(results.Count, Is.EqualTo(1));
+		}
 	}
 }

src/NHibernate.Test/NHSpecificTest/GH3327/Fixture.cs

@@ -48,5 +48,21 @@ WHERE NOT (
 					)");
 			Assert.That(q.List()[0], Is.EqualTo(0));
 		}
+
+		[Test]
+		public void NotNotExistsNegated()
+		{
+			using var log = new SqlLogSpy();
+			using var session = OpenSession();
+			var results = session.CreateQuery(
+				@"SELECT COUNT(ROOT.Id)
+					FROM Entity AS ROOT
+					WHERE NOT (
+						NOT EXISTS (FROM ChildEntity AS CHILD WHERE CHILD.Parent = ROOT)
+						AND NOT ROOT.Name = 'Parent'
+					)").List();
+			Assert.That(log.GetWholeLog(), Does.Not.Contains(" NOT ").IgnoreCase);
+			Assert.That(results.Count, Is.EqualTo(1));
+		}
 	}
 }

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

@@ -300,10 +300,8 @@ public IASTNode NegateNode(IASTNode node)
 					node.Type = BETWEEN;
 					node.Text = "{not}" + node.Text;
 					return node;	// (NOT (NOT_BETWEEN a b) ) => (BETWEEN a b)
-				/* This can never happen because this rule will always eliminate the child NOT.
-							case NOT:
-								return x.getFirstChild();			// (NOT (NOT x) ) => (x)
-				*/
+				case NOT:
+					return node.GetChild(0);	// (NOT (NOT x) ) => (x)
 				default:
 					IASTNode not = (IASTNode) TreeAdaptor.Create(NOT, "not");
 					not.AddChild(node);