do not auto-apply -OrNull combinator to pure window functions

src/AggregateFunctions/AggregateFunctionFactory.cpp

@@ -88,6 +88,15 @@ AggregateFunctionPtr AggregateFunctionFactory::get(
 
         AggregateFunctionPtr nested_function = getImpl(
             name, nested_types, nested_parameters, out_properties, has_null_arguments);
+
+        // Pure window functions are not real aggregate functions. Applying
+        // combinators doesn't make sense for them, they must handle the
+        // nullability themselves.
+        if (nested_function->asWindowFunction())
+        {
+            return nested_function;
+        }
+
         return combinator->transformAggregateFunction(nested_function, out_properties, type_without_low_cardinality, parameters);
     }
 

tests/queries/0_stateless/01591_window_functions.reference

@@ -1054,6 +1054,10 @@ settings max_block_size = 3;
 13	2	10	12	10	10	143
 14	2	10	13	10	10	154
 15	3	15	0	15	15	15
+-- careful with auto-application of Null combinator
+select lagInFrame(toNullable(1)) over ();
+0
+select lagInFrameOrNull(1) over (); -- { serverError 36 }
 -- case-insensitive SQL-standard synonyms for any and anyLast
 select
     number,

tests/queries/0_stateless/01591_window_functions.sql

@@ -376,6 +376,10 @@ order by number
 settings max_block_size = 3;
 ;
 
+-- careful with auto-application of Null combinator
+select lagInFrame(toNullable(1)) over ();
+select lagInFrameOrNull(1) over (); -- { serverError 36 }
+
 -- case-insensitive SQL-standard synonyms for any and anyLast
 select
     number,