MDEV-18886 JSON_ARRAY() does not recognise JSON argument.

JSON_ARRAY and JSON_OBJECT functions with no arguments now get the
connection charset. Item_func_convert_charset returns the correct
is_json() flag.

mysql-test/r/func_json.result

@@ -830,3 +830,15 @@ SET sql_mode=default;
 select JSON_VALID( '{"a":1]' );
 JSON_VALID( '{"a":1]' )
 0
+#
+# MDEV-18886 JSON_ARRAY() does not recognise JSON argument.
+#
+SELECT JSON_ARRAY(_UTF8 'str', JSON_OBJECT(_LATIN1 'plugin', _LATIN1'unix_socket'));
+JSON_ARRAY(_UTF8 'str', JSON_OBJECT(_LATIN1 'plugin', _LATIN1'unix_socket'))
+["str", {"plugin": "unix_socket"}]
+SELECT CHARSET(JSON_ARRAY());
+CHARSET(JSON_ARRAY())
+latin1
+SELECT CHARSET(JSON_OBJECT());
+CHARSET(JSON_OBJECT())
+latin1

mysql-test/t/func_json.test

@@ -487,3 +487,10 @@ SET sql_mode=default;
 
 select JSON_VALID( '{"a":1]' );
 
+--echo #
+--echo # MDEV-18886 JSON_ARRAY() does not recognise JSON argument.
+--echo #
+SELECT JSON_ARRAY(_UTF8 'str', JSON_OBJECT(_LATIN1 'plugin', _LATIN1'unix_socket'));
+SELECT CHARSET(JSON_ARRAY());
+SELECT CHARSET(JSON_OBJECT());
+

sql/item_jsonfunc.cc

@@ -1488,9 +1488,10 @@ bool Item_func_json_array::fix_length_and_dec()
 
   if (arg_count == 0)
   {
-    collation.set(&my_charset_utf8_general_ci,
+    THD* thd= current_thd;
+    collation.set(thd->variables.collation_connection,
                   DERIVATION_COERCIBLE, MY_REPERTOIRE_ASCII);
-    tmp_val.set_charset(&my_charset_utf8_general_ci);
+    tmp_val.set_charset(thd->variables.collation_connection);
     max_length= 2;
     return FALSE;
   }

sql/item_strfunc.h

@@ -1192,6 +1192,7 @@ class Item_func_conv_charset :public Item_str_func
              (cs->state & MY_CS_UNICODE));
     }
   }
+  bool is_json_type() { return args[0]->is_json_type(); }
   String *val_str(String *);
   longlong val_int()
   {