test: add sqllogictest coverage for UDWF return types in information_…

datafusion/sqllogictest/test_files/information_schema.slt

@@ -812,6 +812,49 @@ select is_deterministic from information_schema.routines where routine_name = 'n
 ----
 false
 
+# Test window function return types in information_schema.routines
+# Before PR #20079, window functions would show NULL for data_type
+# This test verifies that UDWF return types are correctly reported
+
+query TTT
+select routine_name, data_type, function_type from information_schema.routines where routine_name = 'row_number';
+----
+row_number NULL WINDOW
+
+query TTT
+select routine_name, data_type, function_type from information_schema.routines where routine_name = 'rank';
+----
+rank NULL WINDOW
+
+query TTT
+select routine_name, data_type, function_type from information_schema.routines where routine_name = 'dense_rank';
+----
+dense_rank NULL WINDOW
+
+query TTT
+select routine_name, data_type, function_type from information_schema.routines where routine_name = 'lag';
+----
+lag NULL WINDOW
+
+query TTT
+select routine_name, data_type, function_type from information_schema.routines where routine_name = 'lead';
+----
+lead NULL WINDOW
+
+# Test that window functions return types match their actual behavior
+# row_number, rank, dense_rank all return NULL (no return type in signature)
+# lag and lead return types depend on their input (shown as NULL in generic signature)
+query TTTB
+select routine_name, data_type, function_type, is_deterministic
+from information_schema.routines
+where function_type = 'WINDOW'
+  and (routine_name = 'row_number' OR routine_name = 'rank' OR routine_name = 'dense_rank')
+order by routine_name;
+----
+dense_rank NULL WINDOW true
+rank NULL WINDOW true
+row_number NULL WINDOW true
+
 # test every function type are included in the result
 query TTTITTTTBI
 select * from information_schema.parameters where specific_name = 'date_trunc' OR specific_name = 'string_agg' OR specific_name = 'rank' ORDER BY specific_name, rid, data_type;