Fixed bug #36488: regexp returns false matches, concatenating

                  with previous rows.

The WHERE clause containing expression:
  CONCAT(empty_field1, empty_field2, ..., 'literal constant', ...)
    REGEXP 'regular expression'
may return wrong matches.

Optimization of the CONCAT function has been fixed.

mysql-test/r/func_concat.result

@@ -82,3 +82,10 @@ a
 1234562
 x
 drop table t1;
+CREATE TABLE t1 (c1 varchar(100), c2 varchar(100));
+INSERT INTO t1 VALUES ('',''), ('','First'), ('Random','Random');
+SELECT * FROM t1 WHERE CONCAT(c1,' ',c2) REGEXP 'First.*';
+c1	c2
+	First
+DROP TABLE t1;
+# End of 5.0 tests

mysql-test/t/func_concat.test

@@ -68,3 +68,13 @@ create table t1(f1 varchar(6)) charset=utf8;
 insert into t1 values ("123456");
 select concat(f1, 2) a from t1 union select 'x' a from t1;
 drop table t1;
+
+#
+# Bug #36488: regexp returns false matches, concatenating with previous rows
+#
+CREATE TABLE t1 (c1 varchar(100), c2 varchar(100));
+INSERT INTO t1 VALUES ('',''), ('','First'), ('Random','Random');
+SELECT * FROM t1 WHERE CONCAT(c1,' ',c2) REGEXP 'First.*';
+DROP TABLE t1;
+
+--echo # End of 5.0 tests

sql/item_strfunc.cc

@@ -294,6 +294,12 @@ String *Item_func_concat::val_str(String *str)
     {
       if (!(res=args[i]->val_str(str)))
 	goto null;
+      /*
+       CONCAT accumulates its result in the result of its the first
+       non-empty argument. Because of this we need is_const to be 
+       evaluated only for it.
+      */
+      is_const= args[i]->const_item() || !args[i]->used_tables();
     }
     else
     {