Bug#50888 valgrind warnings in Field_timestamp::val_str

Ensure that we store the correct cached_field_type whenever we cache Field items
(in this case it allows us to compare dates as dates, rather than strings)

mysql-test/r/type_timestamp.result

@@ -503,3 +503,19 @@ f1	f2-f3
 5	0
 DROP TABLE t1;
 End of 5.0 tests
+
+Bug#50888 valgrind warnings in Field_timestamp::val_str
+
+SET TIMESTAMP=0;
+CREATE TABLE t1(a timestamp);
+INSERT INTO t1 VALUES ('2008-02-23 09:23:45'), ('2010-03-05 11:08:02');
+FLUSH TABLES t1;
+SELECT MAX(a) FROM t1;
+MAX(a)
+2010-03-05 11:08:02
+SELECT a FROM t1;
+a
+2008-02-23 09:23:45
+2010-03-05 11:08:02
+DROP TABLE t1;
+End of Bug#50888

mysql-test/t/type_timestamp.test

@@ -346,3 +346,17 @@ SELECT f1,f2-f3 FROM t1;
 DROP TABLE t1;
 
 --echo End of 5.0 tests
+
+--echo
+--echo Bug#50888 valgrind warnings in Field_timestamp::val_str
+--echo
+
+SET TIMESTAMP=0;
+CREATE TABLE t1(a timestamp);
+INSERT INTO t1 VALUES ('2008-02-23 09:23:45'), ('2010-03-05 11:08:02');
+FLUSH TABLES t1;
+SELECT MAX(a) FROM t1;
+SELECT a FROM t1;
+DROP TABLE t1;
+
+--echo End of Bug#50888

sql/item.h

@@ -3122,14 +3122,16 @@ class Item_cache: public Item_basic_constant
   bool value_cached;
 public:
   Item_cache():
-    example(0), used_table_map(0), cached_field(0), cached_field_type(MYSQL_TYPE_STRING),
+    example(0), used_table_map(0), cached_field(0),
+    cached_field_type(MYSQL_TYPE_STRING),
     value_cached(0)
   {
     fixed= 1; 
     null_value= 1;
   }
   Item_cache(enum_field_types field_type_arg):
-    example(0), used_table_map(0), cached_field(0), cached_field_type(field_type_arg),
+    example(0), used_table_map(0), cached_field(0),
+    cached_field_type(field_type_arg),
     value_cached(0)
   {
     fixed= 1;
@@ -3233,10 +3235,9 @@ class Item_cache_str: public Item_cache
   
 public:
   Item_cache_str(const Item *item) :
-    Item_cache(), value(0),
+    Item_cache(item->field_type()), value(0),
     is_varbinary(item->type() == FIELD_ITEM &&
-                 ((const Item_field *) item)->field->type() ==
-                   MYSQL_TYPE_VARCHAR &&
+                 cached_field_type == MYSQL_TYPE_VARCHAR &&
                  !((const Item_field *) item)->field->has_charset())
   {}
   double val_real();