Bug#35804794: mysqld assertion failure in Query_block::replace_subquery_in_expr

The fix for bug#35621842 revealed a case of missing error propagation
in Item_bool_func2::replace_scalar_subquery(), when transforming a
scalar subquery into a join with a derived table.

Fixed by adding proper error propagation.

Change-Id: I8e4ed26098b9ea765e8daaa086e6a933d832e01b

Author: roylyseng

mysql-test/r/subquery_table_to_derived.result

@@ -497,3 +497,11 @@ c0
 2
 DROP PREPARE p;
 DROP TABLE t1;
+# Bug#35804794: mysqld assertion failure in
+#               Query_block::replace_subquery_in_expr
+CREATE TABLE t1 (col varchar(1));
+SELECT col
+FROM t1
+WHERE col >= (SELECT MAX(CONCAT('nz' COLLATE utf8mb4_la_0900_as_cs)) FROM t1);
+ERROR HY000: Illegal mix of collations (utf8mb4_0900_ai_ci,IMPLICIT) and (utf8mb4_la_0900_as_cs,IMPLICIT) for operation '>='
+DROP TABLE t1;

mysql-test/t/subquery_table_to_derived.test

@@ -356,3 +356,15 @@ EXECUTE p USING @n;
 DROP PREPARE p;
 
 DROP TABLE t1;
+
+--echo # Bug#35804794: mysqld assertion failure in
+--echo #               Query_block::replace_subquery_in_expr
+
+CREATE TABLE t1 (col varchar(1));
+
+--error ER_CANT_AGGREGATE_2COLLATIONS
+SELECT col
+FROM t1
+WHERE col >= (SELECT MAX(CONCAT('nz' COLLATE utf8mb4_la_0900_as_cs)) FROM t1);
+
+DROP TABLE t1;

sql/item_cmpfunc.cc

@@ -857,7 +857,9 @@ Item *Item_func_like::replace_scalar_subquery(uchar *) {
 }
 
 Item *Item_bool_func2::replace_scalar_subquery(uchar *) {
-  (void)set_cmp_func();
+  if (set_cmp_func()) {
+    return nullptr;
+  }
   return this;
 }
 

sql/item_subselect.cc

@@ -3027,7 +3027,7 @@ Item *Item_singlerow_subselect::replace_scalar_subquery(uchar *arg) {
     result = new (current_thd->mem_root)
         Item_ref(&info->m_outer_query_block->context, ref, scalar_item->db_name,
                  scalar_item->table_name, scalar_item->field_name);
-    // nullptr is error, but no separate return needed here
+    if (result == nullptr) return nullptr;
   } else {
     result = scalar_item;
   }