MDEV-7792 - SQL Parsing Error - UNION AND ORDER BY WITH JOIN

ORDER BY against union may confuse name resolution context, causing valid
SQL statements to fail.

The purpose of context change was presumably intended for the duration of
gathering field list for ORDER BY. However it isn't actually required (name
resolution context is never accessed by the latter).

See also alternative solution (in MySQL 5.7): 92145b95.

mysql-test/r/parser.result

@@ -643,3 +643,10 @@ CREATE TRIGGER trigger1 BEFORE INSERT ON t1 FOR EACH ROW
 SET default_storage_engine = NEW.INNODB;
 ERROR 42S22: Unknown column 'INNODB' in 'NEW'
 DROP TABLE t1;
+#
+# MDEV-7792 - SQL Parsing Error - UNION AND ORDER BY WITH JOIN
+#
+CREATE TABLE t1(a INT);
+SELECT * FROM t1 JOIN ((SELECT 1 AS b) UNION ALL (SELECT 2 AS b) ORDER BY b DESC) s1 WHERE a=1;
+a	b
+DROP TABLE t1;

mysql-test/t/parser.test

@@ -758,3 +758,10 @@ CREATE TABLE t1 (s VARCHAR(100));
 CREATE TRIGGER trigger1 BEFORE INSERT ON t1 FOR EACH ROW
 SET default_storage_engine = NEW.INNODB;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-7792 - SQL Parsing Error - UNION AND ORDER BY WITH JOIN
+--echo #
+CREATE TABLE t1(a INT);
+SELECT * FROM t1 JOIN ((SELECT 1 AS b) UNION ALL (SELECT 2 AS b) ORDER BY b DESC) s1 WHERE a=1;
+DROP TABLE t1;

sql/sql_yacc.yy

@@ -11336,7 +11336,6 @@ order_clause:
                */
                DBUG_ASSERT(sel->master_unit()->fake_select_lex);
                lex->current_select= sel->master_unit()->fake_select_lex;
-               lex->push_context(&lex->current_select->context, thd->mem_root);
              }
           }
           order_list