Fix parameter detection for expression arguments (#2542)

Fixes #2537

Author: maca88

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

@@ -707,5 +707,30 @@ public async Task NullComparedToMemberInitExpressionInWhereClauseAsync()
 
 			Assert.That(result.Count, Is.EqualTo(45));
 		}
+
+		public class Specification<T>
+		{
+			private Expression<Func<T, bool>> _expression;
+
+			public Specification(Expression<Func<T, bool>> expression)
+			{
+				_expression = expression;
+			}
+
+			public static implicit operator Expression<Func<T, bool>>(Specification<T> specification)
+			{
+				return specification._expression;
+			}
+		}
+
+		[Test]
+		public async Task ImplicitConversionInsideWhereSubqueryExpressionAsync()
+		{
+			if (!Dialect.SupportsScalarSubSelects)
+				Assert.Ignore(Dialect.GetType().Name + " does not support scalar sub-queries");
+
+			var spec = new Specification<Order>(x => x.Freight > 1000);
+			await (db.Orders.Where(o => db.Orders.Where(spec).Any(x => x.OrderId == o.OrderId)).ToListAsync());
+		}
 	}
 }

src/NHibernate.Test/Linq/WhereSubqueryTests.cs

@@ -788,5 +788,30 @@ public void NullComparedToMemberInitExpressionInWhereClause()
 
 			Assert.That(result.Count, Is.EqualTo(45));
 		}
+
+		public class Specification<T>
+		{
+			private Expression<Func<T, bool>> _expression;
+
+			public Specification(Expression<Func<T, bool>> expression)
+			{
+				_expression = expression;
+			}
+
+			public static implicit operator Expression<Func<T, bool>>(Specification<T> specification)
+			{
+				return specification._expression;
+			}
+		}
+
+		[Test]
+		public void ImplicitConversionInsideWhereSubqueryExpression()
+		{
+			if (!Dialect.SupportsScalarSubSelects)
+				Assert.Ignore(Dialect.GetType().Name + " does not support scalar sub-queries");
+
+			var spec = new Specification<Order>(x => x.Freight > 1000);
+			db.Orders.Where(o => db.Orders.Where(spec).Any(x => x.OrderId == o.OrderId)).ToList();
+		}
 	}
 }

src/NHibernate/Linq/Visitors/NhPartialEvaluatingExpressionVisitor.cs

@@ -157,7 +157,9 @@ private Expression EvaluateSubtree(Expression subtree)
 
 		private bool ContainsVariable(Expression expression)
 		{
-			if (!(expression is UnaryExpression unaryExpression))
+			if (!(expression is UnaryExpression unaryExpression) ||
+				// Avoid detecting expression variables as parameters
+				typeof(Expression).IsAssignableFrom(expression.Type))
 			{
 				return false;
 			}