MDEV-16359 wrong result (extra rows) on the query with UNION and brackets

Due to an obvious bug in st_select_lex_unit::reset_distinct() a union
without duplicates of more than 2 selects could return a result set
containing duplicate rows if this union was enclosed in ().

mysql-test/main/brackets.result

@@ -207,4 +207,16 @@ a	b
 1	100
 3	30
 drop table t1;
+#
+# MDEV-16359: union with 3 selects in brackets
+#
+select 1 union select 1 union select 1;
+1
+1
+(select 1 union select 1 union select 1);
+1
+1
+((select 1) union (select 1) union (select 1));
+1
+1
 # End of 10.4 tests

mysql-test/main/brackets.test

@@ -80,5 +80,13 @@ select a,b from t1 order by 1 limit 3;
 
 drop table t1;
 
+--echo #
+--echo # MDEV-16359: union with 3 selects in brackets
+--echo #
+
+select 1 union select 1 union select 1;
+(select 1 union select 1 union select 1);
+((select 1) union (select 1) union (select 1));
+
 --echo # End of 10.4 tests
 

sql/sql_lex.cc

@@ -8763,7 +8763,6 @@ void st_select_lex_unit::reset_distinct()
     if (sl->distinct)
     {
       union_distinct= sl;
-      return;
     }
   }
 }