cleanup: extend Item_cache::get_cache() to accept f_type

Do not assume that it's always item->field_type() - this is not the case
in temporal comparisons (e.g. when comparing DATETIME column with a TIME
literal).

sql/item.cc

@@ -9543,12 +9543,6 @@ int stored_field_cmp_to_item(THD *thd, Field *field, Item *item)
   return 0;
 }
 
-Item_cache* Item_cache::get_cache(THD *thd, const Item *item)
-{
-  return get_cache(thd, item, item->cmp_type());
-}
-
-
 /**
   Get a cache item of given type.
 
@@ -9559,12 +9553,12 @@ Item_cache* Item_cache::get_cache(THD *thd, const Item *item)
 */
 
 Item_cache* Item_cache::get_cache(THD *thd, const Item *item,
-                                  const Item_result type)
+                         const Item_result type, const enum_field_types f_type)
 {
   MEM_ROOT *mem_root= thd->mem_root;
   switch (type) {
   case INT_RESULT:
-    return new (mem_root) Item_cache_int(thd, item->field_type());
+    return new (mem_root) Item_cache_int(thd, f_type);
   case REAL_RESULT:
     return new (mem_root) Item_cache_real(thd);
   case DECIMAL_RESULT:
@@ -9574,7 +9568,7 @@ Item_cache* Item_cache::get_cache(THD *thd, const Item *item,
   case ROW_RESULT:
     return new (mem_root) Item_cache_row(thd);
   case TIME_RESULT:
-    return new (mem_root) Item_cache_temporal(thd, item->field_type());
+    return new (mem_root) Item_cache_temporal(thd, f_type);
   }
   return 0;                                     // Impossible
 }

sql/item.h

@@ -5551,8 +5551,17 @@ class Item_cache: public Item_basic_constant,
   enum Item_result cmp_type () const
   { return Type_handler_hybrid_field_type::cmp_type(); }
 
-  static Item_cache* get_cache(THD *thd, const Item *item);
-  static Item_cache* get_cache(THD *thd, const Item* item, const Item_result type);
+  static Item_cache* get_cache(THD *thd, const Item* item,
+                         const Item_result type, const enum_field_types f_type);
+  static Item_cache* get_cache(THD *thd, const Item* item,
+                         const Item_result type)
+  {
+    return get_cache(thd, item, type, item->field_type());
+  }
+  static Item_cache* get_cache(THD *thd, const Item *item)
+  {
+    return get_cache(thd, item, item->cmp_type());
+  }
   virtual void keep_array() {}
   virtual void print(String *str, enum_query_type query_type);
   bool eq_def(const Field *field) 

sql/item_row.h

@@ -90,8 +90,7 @@ class Item_row: public Item,
   Item_result cmp_type() const { return ROW_RESULT; }
   enum_field_types field_type() const
   {
-    DBUG_ASSERT(0);
-    return MYSQL_TYPE_DOUBLE;
+    return MYSQL_TYPE_NULL;
   }
   void update_used_tables()
   {