Fix bug#12266 GROUP BY DATE(LEFT(column,8)) returns result strings with reduced

length.

When temporary field created for DATE(LEFT(column,8)) expression, max_length
value is taken from Item_date_typecast, and it is getting it from underlaid
Item_func_left and it's max_length is 8 in given expression. And all this
results in stripping last 2 digits.

To Item_date_typecast class added its own fix_length_and_dec() function 
that sets max_length value to 10, which is proper for DATE field.

mysql-test/r/group_by.result

@@ -757,3 +757,10 @@ SELECT n+1 AS n FROM t1 GROUP BY n;
 n
 2
 DROP TABLE t1;
+create table t1 (f1 date);
+insert into t1 values('2005-06-06');
+insert into t1 values('2005-06-06');
+select date(left(f1+0,8)) from t1 group by 1;
+date(left(f1+0,8))
+2005-06-06
+drop table t1;

mysql-test/t/group_by.test

@@ -590,4 +590,14 @@ INSERT INTO t1 VALUES (1);
 SELECT n+1 AS n FROM t1 GROUP BY n;
 DROP TABLE t1;
 
+#
+# Bug #12266 GROUP BY expression on DATE column produces result with
+#            reduced length
+#
+create table t1 (f1 date);
+insert into t1 values('2005-06-06');
+insert into t1 values('2005-06-06'); 
+select date(left(f1+0,8)) from t1 group by 1;
+drop table t1;
+
 # End of 4.1 tests

sql/item_timefunc.h

@@ -709,6 +709,12 @@ public:
   {
     return (new Field_date(maybe_null, name, t_arg, &my_charset_bin));
   }  
+  void fix_length_and_dec()
+  {
+    collation.set(&my_charset_bin);
+    max_length= 10;
+    maybe_null= 1;
+  }
 };