Fix parameter detection for custom hql method generators

src/NHibernate.Test/Async/Linq/CustomExtensionsExample.cs

@@ -14,6 +14,7 @@
 using System.Reflection;
 using System.Text.RegularExpressions;
 using NHibernate.Cfg;
+using NHibernate.DomainModel.Northwind.Entities;
 using NHibernate.Hql.Ast;
 using NHibernate.Linq.Functions;
 using NHibernate.Linq.Visitors;
@@ -33,6 +34,14 @@ protected override void Configure(NHibernate.Cfg.Configuration configuration)
 			configuration.LinqToHqlGeneratorsRegistry<MyLinqToHqlGeneratorsRegistry>();
 		}
 
+		[Test]
+		public async Task CanUseObjectEqualsAsync()
+		{
+			var users = await (db.Users.Where(o => ((object) EnumStoredAsString.Medium).Equals(o.NullableEnum1)).ToListAsync());
+			Assert.That(users.Count, Is.EqualTo(2));
+			Assert.That(users.All(c => c.NullableEnum1 == EnumStoredAsString.Medium), Is.True);
+		}
+
 		[Test]
 		public async Task CanUseMyCustomExtensionAsync()
 		{

src/NHibernate.Test/Linq/CustomExtensionsExample.cs

@@ -4,6 +4,7 @@
 using System.Reflection;
 using System.Text.RegularExpressions;
 using NHibernate.Cfg;
+using NHibernate.DomainModel.Northwind.Entities;
 using NHibernate.Hql.Ast;
 using NHibernate.Linq.Functions;
 using NHibernate.Linq.Visitors;
@@ -30,6 +31,7 @@ public MyLinqToHqlGeneratorsRegistry():base()
 		{
 			RegisterGenerator(ReflectHelper.GetMethodDefinition(() => MyLinqExtensions.IsLike(null, null)),
 							  new IsLikeGenerator());
+			RegisterGenerator(ReflectHelper.GetMethodDefinition(() => new object().Equals(null)), new ObjectEqualsGenerator());
 		}
 	}
 
@@ -48,6 +50,21 @@ public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject,
 		}
 	}
 
+	public class ObjectEqualsGenerator : BaseHqlGeneratorForMethod
+	{
+		public ObjectEqualsGenerator()
+		{
+			SupportedMethods = new[] { ReflectHelper.GetMethodDefinition(() => new object().Equals(null)) };
+		}
+
+		public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject,
+		                                     ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
+		{
+			return treeBuilder.Equality(visitor.Visit(targetObject).AsExpression(),
+			                        visitor.Visit(arguments[0]).AsExpression());
+		}
+	}
+
 	[TestFixture]
 	public class CustomExtensionsExample : LinqTestCase
 	{
@@ -56,6 +73,14 @@ protected override void Configure(NHibernate.Cfg.Configuration configuration)
 			configuration.LinqToHqlGeneratorsRegistry<MyLinqToHqlGeneratorsRegistry>();
 		}
 
+		[Test]
+		public void CanUseObjectEquals()
+		{
+			var users = db.Users.Where(o => ((object) EnumStoredAsString.Medium).Equals(o.NullableEnum1)).ToList();
+			Assert.That(users.Count, Is.EqualTo(2));
+			Assert.That(users.All(c => c.NullableEnum1 == EnumStoredAsString.Medium), Is.True);
+		}
+
 		[Test]
 		public void CanUseMyCustomExtension()
 		{

src/NHibernate/Linq/Visitors/ParameterTypeLocator.cs

@@ -156,6 +156,11 @@ private static IType GetParameterType(
 				return candidateType;
 			}
 
+			if (visitor.NotGuessableConstants.Contains(constantExpression))
+			{
+				return null;
+			}
+
 			// No related MemberExpressions was found, guess the type by value or its type when null.
 			// When a numeric parameter is compared to different columns with different types (e.g. Where(o => o.Single >= singleParam || o.Double <= singleParam))
 			// do not change the parameter type, but instead cast the parameter when comparing with different column types.
@@ -166,10 +171,13 @@ private static IType GetParameterType(
 
 		private class ConstantTypeLocatorVisitor : RelinqExpressionVisitor
 		{
+			private bool _hqlGenerator;
 			private readonly bool _removeMappedAsCalls;
 			private readonly System.Type _targetType;
 			private readonly IDictionary<ConstantExpression, NamedParameter> _parameters;
 			private readonly ISessionFactoryImplementor _sessionFactory;
+			private readonly ILinqToHqlGeneratorsRegistry _functionRegistry;
+			public readonly HashSet<ConstantExpression> NotGuessableConstants = new HashSet<ConstantExpression>();
 			public readonly Dictionary<ConstantExpression, IType> ConstantExpressions =
 				new Dictionary<ConstantExpression, IType>();
 			public readonly Dictionary<NamedParameter, HashSet<ConstantExpression>> ParameterConstants =
@@ -187,6 +195,7 @@ public ConstantTypeLocatorVisitor(
 				_targetType = targetType;
 				_sessionFactory = sessionFactory;
 				_parameters = parameters;
+				_functionRegistry = sessionFactory.Settings.LinqToHqlGeneratorsRegistry;
 			}
 
 			protected override Expression VisitBinary(BinaryExpression node)
@@ -257,6 +266,16 @@ protected override Expression VisitMethodCall(MethodCallExpression node)
 					return node;
 				}
 
+				// For hql method generators we do not want to guess the parameter type here, let hql logic figure it out.
+				if (_functionRegistry.TryGetGenerator(node.Method, out _))
+				{
+					var origHqlGenerator = _hqlGenerator;
+					_hqlGenerator = true;
+					var expression = base.VisitMethodCall(node);
+					_hqlGenerator = origHqlGenerator;
+					return expression;
+				}
+
 				return base.VisitMethodCall(node);
 			}
 
@@ -267,6 +286,11 @@ protected override Expression VisitConstant(ConstantExpression node)
 					return node;
 				}
 
+				if (_hqlGenerator)
+				{
+					NotGuessableConstants.Add(node);
+				}
+
 				RelatedExpressions.Add(node, new HashSet<Expression>());
 				ConstantExpressions.Add(node, null);
 				if (!ParameterConstants.TryGetValue(param, out var set))