Fix LINQ projection of nullable enum with list

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

@@ -9,6 +9,7 @@
 
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using NHibernate.Cfg.MappingSchema;
 using NHibernate.Mapping.ByCode;
@@ -50,9 +51,24 @@ protected override HbmMapping GetMappings()
 						m.Type(_enumType);
 						m.Formula($"(case when Enum1 = {_unspecifiedValue} then null else Enum1 end)");
 					});
+					rc.Bag(x => x.Children, m => 
+						{
+							m.Cascade(Mapping.ByCode.Cascade.All);
+							m.Inverse(true);
+						},
+						a => a.OneToMany()
+					);
 					rc.ManyToOne(x => x.Other, m => m.Cascade(Mapping.ByCode.Cascade.All));
 				});
 
+			mapper.Class<EnumEntityChild>(
+				rc =>
+				{
+					rc.Table("EnumEntityChild");
+					rc.Id(x => x.Id, m => m.Generator(Generators.Guid));
+					rc.Property(x => x.Name);
+				});
+
 			return mapper.CompileMappingForAllExplicitlyAddedEntities();
 		}
 
@@ -184,5 +200,25 @@ public async Task CanQueryComplexExpressionOnTestEnumAsync()
 				Assert.That(query.Count, Is.EqualTo(0));
 			}
 		}
+
+		[Test]
+		public async Task CanProjectWithListTransformationAsync()
+		{
+			using (var session = OpenSession())
+			using (var trans = session.BeginTransaction())
+			{
+				var entities = session.Query<EnumEntity>();
+
+				var query = await (entities.Select(user => new
+				{
+					user.Name,
+					simple = user.Enum1,
+					children = user.Children,
+					nullableEnum1IsLarge = user.NullableEnum1 == TestEnum.Large
+				}).ToListAsync());
+
+				Assert.That(query.Count, Is.EqualTo(10));
+			}
+		}
 	}
 }

src/NHibernate.Test/Linq/EnumTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using NHibernate.Cfg.MappingSchema;
 using NHibernate.Mapping.ByCode;
@@ -37,9 +38,24 @@ protected override HbmMapping GetMappings()
 						m.Type(_enumType);
 						m.Formula($"(case when Enum1 = {_unspecifiedValue} then null else Enum1 end)");
 					});
+					rc.Bag(x => x.Children, m => 
+						{
+							m.Cascade(Mapping.ByCode.Cascade.All);
+							m.Inverse(true);
+						},
+						a => a.OneToMany()
+					);
 					rc.ManyToOne(x => x.Other, m => m.Cascade(Mapping.ByCode.Cascade.All));
 				});
 
+			mapper.Class<EnumEntityChild>(
+				rc =>
+				{
+					rc.Table("EnumEntityChild");
+					rc.Id(x => x.Id, m => m.Generator(Generators.Guid));
+					rc.Property(x => x.Name);
+				});
+
 			return mapper.CompileMappingForAllExplicitlyAddedEntities();
 		}
 
@@ -171,6 +187,26 @@ public void CanQueryComplexExpressionOnTestEnum()
 				Assert.That(query.Count, Is.EqualTo(0));
 			}
 		}
+
+		[Test]
+		public void CanProjectWithListTransformation()
+		{
+			using (var session = OpenSession())
+			using (var trans = session.BeginTransaction())
+			{
+				var entities = session.Query<EnumEntity>();
+
+				var query = entities.Select(user => new
+				{
+					user.Name,
+					simple = user.Enum1,
+					children = user.Children,
+					nullableEnum1IsLarge = user.NullableEnum1 == TestEnum.Large
+				}).ToList();
+
+				Assert.That(query.Count, Is.EqualTo(10));
+			}
+		}
 	}
 
 	public class EnumEntity
@@ -181,7 +217,17 @@ public class EnumEntity
 		public virtual TestEnum Enum1 { get; set; }
 		public virtual TestEnum? NullableEnum1 { get; set; }
 
+		public virtual int? NullableInt { get; set; }
+
 		public virtual EnumEntity Other { get; set; }
+
+		public virtual IList<EnumEntityChild> Children { get; set; } = new List<EnumEntityChild>();
+	}
+
+	public class EnumEntityChild
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
 	}
 
 	public enum TestEnum

src/NHibernate/Linq/NestedSelects/SelectClauseRewriter.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq.Expressions;
+using NHibernate.Util;
 using Remotion.Linq.Clauses.Expressions;
 using Remotion.Linq.Parsing;
 
@@ -41,6 +42,10 @@ public override Expression Visit(Expression expression)
 		protected override Expression VisitUnary(UnaryExpression node)
 		{
 			if (node.NodeType == ExpressionType.Convert &&
+				// We can skip a convert node only when the underlying types are equal otherwise it
+				// will throw an exception when trying to convert the value from an object
+				// (e.g. (int?)(Enum?) input[0] -> (Enum?) cast cannot be skipped)
+				node.Type.UnwrapIfNullable() == node.Operand.Type.UnwrapIfNullable() &&
 				(node.Operand is MemberExpression || node.Operand is QuerySourceReferenceExpression))
 			{
 				return AddAndConvertExpression(node.Operand, node.Type);
@@ -75,4 +80,4 @@ private Expression AddAndConvertExpression(Expression expression, System.Type ty
 				type);
 		}
 	}
-}
+}