Fixed mdev-15120 CTE table should not belong to database, that is in use

When identifying a table name the following should be taken into account:
a CTE name cannot be qualified with a database name, otherwise the table
name is considered as the name of a non-CTE table.

mysql-test/r/cte_nonrecursive.result

@@ -1369,3 +1369,13 @@ n_nationkey	n_name	n_regionkey	r_regionkey	r_name
 23	UNITED KINGDOM	3	3	EUROPE
 drop view v;
 drop table region, nation;
+#
+# MDEV-15120: cte name used with database name
+#
+WITH cte AS (SELECT 1 AS a) SELECT test.cte.a FROM test.cte;
+ERROR 42S02: Table 'test.cte' doesn't exist
+CREATE DATABASE db1;
+USE db1;
+WITH cte AS (SELECT 1 AS a) SELECT db1.cte.a FROM db1.cte;
+ERROR 42S02: Table 'db1.cte' doesn't exist
+DROP DATABASE db1;

mysql-test/t/cte_nonrecursive.test

@@ -929,3 +929,18 @@ select * from v;
 
 drop view v;
 drop table region, nation;
+
+--echo #
+--echo # MDEV-15120: cte name used with database name
+--echo #
+
+--error ER_NO_SUCH_TABLE
+WITH cte AS (SELECT 1 AS a) SELECT test.cte.a FROM test.cte;
+
+CREATE DATABASE db1;
+USE db1;
+
+--error ER_NO_SUCH_TABLE
+WITH cte AS (SELECT 1 AS a) SELECT db1.cte.a FROM db1.cte;
+
+DROP DATABASE db1;

sql/sql_cte.cc

@@ -240,7 +240,8 @@ With_element *With_clause::find_table_def(TABLE_LIST *table,
        with_elem= with_elem->next)
   {
     if (my_strcasecmp(system_charset_info, with_elem->query_name->str,
-		      table->table_name.str) == 0)
+		      table->table_name.str) == 0 &&
+        !table->is_fqtn)
     {
       table->set_derived();
       return with_elem;