fix null queries over links to an indexed property (#4460)

CHANGELOG.md

@@ -5,6 +5,7 @@
 
 ### Fixed
 * <How to hit and notice issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
+* Fixed queries for constant null across links to an indexed property not returning matches when the link was null. ([#4460]https://github.com/realm/realm-core/pull/4460), since 5.23.6).
 * Support upgrading from file format 5. ([#7089](https://github.com/realm/realm-cocoa/issues/7089), since v6.0.0)
 * On 32bit devices you may get exception with "No such object" when upgrading to v10.* ([#7314](https://github.com/realm/realm-java/issues/7314), since v10.0.0)
 * The notification worker thread would rerun queries after every commit rather than only commits which modified tables which could effect the query results if the table had any outgoing links to tables not used in the query (since v6.0.0).

src/realm/query_expression.hpp

@@ -4319,7 +4319,15 @@ class Compare : public Expression {
         if (std::is_same_v<TCond, Equal> && m_left_is_const && m_right->has_search_index() &&
             m_right->get_comparison_type() == ExpressionComparisonType::Any) {
             if (m_left_value.is_null()) {
-                m_matches = m_right->find_all(Mixed());
+                const ObjPropertyBase* prop = dynamic_cast<const ObjPropertyBase*>(m_right.get());
+                // when checking for null across links, null links are considered matches,
+                // so we must compute the slow matching even if there is an index.
+                if (!prop || prop->links_exist()) {
+                    return dT;
+                }
+                else {
+                    m_matches = m_right->find_all(Mixed());
+                }
             }
             else {
                 if (m_right->get_type() != m_left_value.get_type()) {

test/test_parser.cpp

@@ -497,6 +497,8 @@ TEST(Parser_basic_serialisation)
     verify_query(test_context, t, "buddy == nil", 4);
     verify_query(test_context, t, "buddy != NULL", 1);
     verify_query(test_context, t, "buddy <> NULL", 1);
+    verify_query(test_context, t, "buddy.name == NULL", 4); // matches null links
+    verify_query(test_context, t, "buddy.age == NULL", 4);
     verify_query(test_context, t, "age > 2", 2);
     verify_query(test_context, t, "!(age >= 2)", 2);
     verify_query(test_context, t, "!(age => 2)", 2);