Creating a new class in_string::Item_string_for_in_vector

and moving set_value() from Item_string to Item_string_for_in_vector,
as set_value() updates the members incompletely
(e.g. does not update max_length),
so it was dangerous to have set_value() available in Item_string.

sql/item.h

@@ -2762,6 +2762,7 @@ public:
     decimals= NOT_FIXED_DEC;
     fixed= 1;
   }
+protected:
   /* Just create an item and do not fill string representation */
   Item_string(CHARSET_INFO *cs, Derivation dv= DERIVATION_COERCIBLE)
     : m_cs_specified(FALSE)
@@ -2772,6 +2773,7 @@ public:
     decimals= NOT_FIXED_DEC;
     fixed= 1;
   }
+public:
   Item_string(const char *name_par, const char *str, uint length,
               CHARSET_INFO *cs, Derivation dv= DERIVATION_COERCIBLE,
               uint repertoire= MY_REPERTOIRE_UNICODE30)
@@ -2785,11 +2787,6 @@ public:
     // it is constant => can be used without fix_fields (and frequently used)
     fixed= 1;
   }
-  void set_value(const String *str)
-  {
-    str_value= *str;
-    collation.set(str->charset());
-  }
   void copy_value(const char *str, uint32 length, CHARSET_INFO *fromcs,
                   CHARSET_INFO *tocs, uint *cnv_errors)
   {

sql/item_cmpfunc.h

@@ -883,6 +883,18 @@ class in_string :public in_vector
 {
   char buff[STRING_BUFFER_USUAL_SIZE];
   String tmp;
+  class Item_string_for_in_vector: public Item_string
+  {
+  public:
+    Item_string_for_in_vector(CHARSET_INFO *cs):
+      Item_string(cs)
+    { }
+    void set_value(const String *str)
+    {
+      str_value= *str;
+      collation.set(str->charset());
+    }
+  };
 public:
   in_string(uint elements,qsort2_cmp cmp_func, CHARSET_INFO *cs);
   ~in_string();
@@ -890,12 +902,12 @@ public:
   uchar *get_value(Item *item);
   Item* create_item()
   { 
-    return new Item_string(collation);
+    return new Item_string_for_in_vector(collation);
   }
   void value_to_item(uint pos, Item *item)
   {    
     String *str=((String*) base)+pos;
-    Item_string *to= (Item_string*)item;
+    Item_string_for_in_vector *to= (Item_string_for_in_vector*) item;
     to->set_value(str);
   }
   Item_result result_type() { return STRING_RESULT; }