Revert "MDEV-11439 No data type JSON, but CAST(something AS JSON) pretends to work"

This reverts commit 1f372cf.
Wasn't supposed to be pushed just yet.

Author: vuvova

mysql-test/r/func_json.result

@@ -402,6 +402,15 @@ json_object("a", json_object("b", "abcd"))
 select json_object("a", '{"b": "abcd"}');
 json_object("a", '{"b": "abcd"}')
 {"a": "{\"b\": \"abcd\"}"}
+select json_object("a", cast('{"b": "abcd"}' as json));
+json_object("a", cast('{"b": "abcd"}' as json))
+{"a": {"b": "abcd"}}
+select cast(NULL AS JSON);
+cast(NULL AS JSON)
+NULL
+select json_depth(cast(NULL as JSON));
+json_depth(cast(NULL as JSON))
+NULL
 select json_depth('[[], {}]');
 json_depth('[[], {}]')
 2

mysql-test/t/func_json.test

@@ -161,10 +161,10 @@ select json_unquote('abc');
 
 select json_object("a", json_object("b", "abcd"));
 select json_object("a", '{"b": "abcd"}');
-#select json_object("a", cast('{"b": "abcd"}' as json));
+select json_object("a", cast('{"b": "abcd"}' as json));
 
-#select cast(NULL AS JSON);
-#select json_depth(cast(NULL as JSON));
+select cast(NULL AS JSON);
+select json_depth(cast(NULL as JSON));
 select json_depth('[[], {}]');
 select json_depth('[[[1,2,3],"s"], {}, []]');
 select json_depth('[10, {"a": 20}]');

sql/sql_yacc.yy

@@ -10714,6 +10714,7 @@ cast_type:
           }
         | cast_type_numeric  { $$= $1; Lex->charset= NULL; }
         | cast_type_temporal { $$= $1; Lex->charset= NULL; }
+        | JSON_SYM           { $$.set(ITEM_CAST_JSON); }
         ;
 
 cast_type_numeric: