MDEV-18284: JSON casting using JSON_COMPACT doesn't always work with values from subqueries

- Cherry-pick 2fcff310 (MDEV-21902)
- Closed PR #1145
Reviewed by: holyfoot@mariadb.com

mysql-test/r/func_json.result

@@ -940,5 +940,23 @@ SELECT JSON_REPLACE( JSON_DETAILED('["x"]'), '$.a', 'xx' );
 JSON_REPLACE( JSON_DETAILED('["x"]'), '$.a', 'xx' )
 ["x"]
 #
+# MDEV-18284 JSON casting using JSON_COMPACT doesn't always work
+#            with values from subqueries
+#
+CREATE TABLE json_test(a JSON, b JSON);
+INSERT INTO json_test VALUES ("[1,2,3]", '{"a":"foo"}');
+SELECT * FROM json_test;
+a	b
+[1,2,3]	{"a":"foo"}
+SELECT json_object("a", json_compact(a), "b", b)
+FROM (SELECT * FROM json_test) AS json_test_values;
+json_object("a", json_compact(a), "b", b)
+{"a": [1,2,3], "b": "{\"a\":\"foo\"}"}
+SELECT json_object("a", json_compact(a), "b", json_compact(b))
+FROM (SELECT * FROM json_test) AS json_test_values;
+json_object("a", json_compact(a), "b", json_compact(b))
+{"a": [1,2,3], "b": {"a":"foo"}}
+DROP TABLE json_test;
+#
 # End of 10.2 tests
 #

mysql-test/t/func_json.test

@@ -560,6 +560,21 @@ SELECT
 
 SELECT JSON_REPLACE( JSON_DETAILED('["x"]'), '$.a', 'xx' );
 
+--echo #
+--echo # MDEV-18284 JSON casting using JSON_COMPACT doesn't always work
+--echo #            with values from subqueries
+--echo #
+
+CREATE TABLE json_test(a JSON, b JSON);
+INSERT INTO json_test VALUES ("[1,2,3]", '{"a":"foo"}');
+SELECT * FROM json_test;
+
+SELECT json_object("a", json_compact(a), "b", b)
+  FROM (SELECT * FROM json_test) AS json_test_values;
+SELECT json_object("a", json_compact(a), "b", json_compact(b))
+  FROM (SELECT * FROM json_test) AS json_test_values;
+DROP TABLE json_test;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/item.h

@@ -4564,6 +4564,7 @@ class Item_ref :public Item_ident
   {
     return ref ? (*ref)->real_item() : this;
   }
+  bool is_json_type() { return (*ref)->is_json_type(); }
   bool walk(Item_processor processor, bool walk_subquery, void *arg)
   { 
     if (ref && *ref)