MDEV-15478: Lost name of a explicitly named CTE column used in

            the non-recursive CTE defined with UNION

The problem appears as the columns of the non-recursive CTE weren't renamed.
The renaming procedure was called for recursive CTEs only.

To fix it in the procedure st_select_lex_unit::prepare
With_element::rename_columns_of_derived_unit is called now for both CTEs:
recursive and non-recursive.

mysql-test/r/cte_nonrecursive.result

@@ -1425,3 +1425,40 @@ a
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 DROP VIEW v1,v2;
+#
+# MDEV-15478: Lost name of a explicitly named CTE column used in
+#             the non-recursive CTE defined with UNION
+#
+CREATE TABLE t1 (x int, y int);
+INSERT INTO t1 VALUES (1,2),(2,7),(3,3);
+WITH cte(a) AS (SELECT 1 UNION SELECT 2) SELECT * FROM cte;
+a
+1
+2
+WITH cte(a) AS (SELECT 1 UNION SELECT 2) SELECT a FROM cte;
+a
+1
+2
+WITH cte(a) AS (SELECT 1 UNION ALL SELECT 1) SELECT a FROM cte;
+a
+1
+1
+WITH cte(a) AS (SELECT x from t1 UNION SELECT 4) SELECT a FROM cte;
+a
+1
+2
+3
+4
+WITH cte(a) AS (SELECT 4 UNION SELECT x FROM t1 UNION SELECT 5)
+SELECT a FROM cte;
+a
+4
+1
+2
+3
+5
+WITH cte(a,b) AS (SELECT 4,5 UNION SELECT 4,3) SELECT a,b FROM cte;
+a	b
+4	5
+4	3
+DROP TABLE t1;

mysql-test/t/cte_nonrecursive.test

@@ -989,3 +989,26 @@ DEALLOCATE PREPARE stmt;
 
 DROP TABLE t1;
 DROP VIEW v1,v2;
+
+--echo #
+--echo # MDEV-15478: Lost name of a explicitly named CTE column used in
+--echo #             the non-recursive CTE defined with UNION
+--echo #
+
+CREATE TABLE t1 (x int, y int);
+INSERT INTO t1 VALUES (1,2),(2,7),(3,3);
+
+WITH cte(a) AS (SELECT 1 UNION SELECT 2) SELECT * FROM cte;
+
+WITH cte(a) AS (SELECT 1 UNION SELECT 2) SELECT a FROM cte;
+
+WITH cte(a) AS (SELECT 1 UNION ALL SELECT 1) SELECT a FROM cte;
+
+WITH cte(a) AS (SELECT x from t1 UNION SELECT 4) SELECT a FROM cte;
+
+WITH cte(a) AS (SELECT 4 UNION SELECT x FROM t1 UNION SELECT 5)
+SELECT a FROM cte;
+
+WITH cte(a,b) AS (SELECT 4,5 UNION SELECT 4,3) SELECT a,b FROM cte;
+
+DROP TABLE t1;

sql/sql_union.cc

@@ -598,7 +598,7 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
       types= first_sl->item_list;
     else if (sl == first_sl)
     {
-      if (is_recursive)
+      if (with_element)
       {
         if (derived->with->rename_columns_of_derived_unit(thd, this))
 	  goto err;