#7874: CONCAT() gives wrong results mixing latin1 field and utf8 string literals

We should not overwrite res if it is returned from a const item.

mysql-test/r/ctype_utf8.result

@@ -849,3 +849,15 @@ utf8_bin	6109
 utf8_bin	61
 utf8_bin	6120
 drop table t1;
+CREATE TABLE t1 (
+user varchar(255) NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+INSERT INTO t1 VALUES ('one'),('two');
+SELECT CHARSET('a');
+CHARSET('a')
+utf8
+SELECT user, CONCAT('<', user, '>') AS c FROM t1;
+user	c
+one	<one>
+two	<two>
+DROP TABLE t1;

mysql-test/t/ctype_utf8.test

@@ -681,3 +681,15 @@ SET collation_connection='utf8_general_ci';
 -- source include/ctype_filesort.inc
 SET collation_connection='utf8_bin';
 -- source include/ctype_filesort.inc
+
+#
+# Bug #7874 CONCAT() gives wrong results mixing
+# latin1 field and utf8 string literals
+#
+CREATE TABLE t1 (
+	user varchar(255) NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+INSERT INTO t1 VALUES ('one'),('two');
+SELECT CHARSET('a');
+SELECT user, CONCAT('<', user, '>') AS c FROM t1;
+DROP TABLE t1;

sql/item_strfunc.cc

@@ -275,7 +275,8 @@ String *Item_func_concat::val_str(String *str)
 			    current_thd->variables.max_allowed_packet);
 	goto null;
       }
-      if (res->alloced_length() >= res->length()+res2->length())
+      if (!args[0]->const_item() && 
+          res->alloced_length() >= res->length()+res2->length())
       {						// Use old buffer
 	res->append(*res2);
       }