Merged (will need a post-merge fix)

sql/item.h:
  Auto merged
sql/item_cmpfunc.cc:
  Auto merged

mysql-test/r/group_by.result

@@ -638,3 +638,15 @@ alias
 1,2
 1
 drop table t1;
+create table t1 (a int);
+insert into t1 values(null);
+select min(a) is null from t1;
+min(a) is null
+1
+select min(a) is null or null from t1;
+min(a) is null or null
+1
+select 1 and min(a) is null from t1;
+1 and min(a) is null
+1
+drop table t1;

mysql-test/t/group_by.test

@@ -465,3 +465,12 @@ select group_concat( distinct col1 ) as alias from t1
 
 drop table t1;
 
+
+#Test for BUG#6976: Aggregate functions have incorrect NULL-ness
+create table t1 (a int);
+insert into t1 values(null);
+select min(a) is null from t1;
+select min(a) is null or null from t1;
+select 1 and min(a) is null from t1;
+drop table t1;
+

sql/item.cc

@@ -1488,14 +1488,13 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
 		   "forward reference in item list");
 	  return -1;
 	}
-
 	Item_ref *rf= (place == IN_HAVING ?
                        new Item_ref(last->ref_pointer_array + counter,
                                     (char *)table_name,
-                                    (char *)field_name) :
+                                    (char *)field_name, this) :
                        new Item_direct_ref(last->ref_pointer_array + counter,
                                            (char *)table_name,
-                                           (char *)field_name));
+                                           (char *)field_name, this));
 	if (!rf)
 	  return 1;
         thd->change_item_tree(ref, rf);

sql/item.h

@@ -835,6 +835,26 @@ public:
     :Item_ident(db_par, table_name_par, field_name_par), ref(0) {}
   Item_ref(Item **item, const char *table_name_par, const char *field_name_par)
     :Item_ident(NullS, table_name_par, field_name_par), ref(item) {}
+  
+  /*
+    This constructor is used when processing GROUP BY and referred Item is 
+    available. We set all properties here because fix_fields() will not be 
+    called for the created Item_ref. (see BUG#6976) 
+    TODO check if we could get rid of *_name_par parameters and if we need to 
+         perform similar initialization for other ctors.
+    TODO we probably fix a superset of problems like in BUG#6658. Check this 
+         with Bar, and if we have a more broader set of problems like this.
+  */
+  Item_ref(Item **item, const char *table_name_par, 
+           const char *field_name_par, Item *src)
+    : Item_ident(NullS, table_name_par, field_name_par), ref(item)
+  {
+    collation.set(src->collation);
+    max_length= src->max_length;
+    decimals=	src->decimals;
+    with_sum_func= src->with_sum_func;
+    maybe_null= src->maybe_null;
+  }
   /* Constructor need to process subselect with temporary tables (see Item) */
   Item_ref(THD *thd, Item_ref *item) :Item_ident(thd, item), ref(item->ref) {}
   enum Type type() const		{ return REF_ITEM; }
@@ -933,6 +953,7 @@ public:
 
 
 class Item_in_subselect;
+
 class Item_ref_null_helper: public Item_ref
 {
 protected:

sql/item_cmpfunc.cc

@@ -2022,7 +2022,8 @@ void Item_cond::split_sum_func(THD *thd, Item **ref_pointer_array,
     {
       Item **ref= li.ref();
       uint el= fields.elements;
-      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name,
+                                   item);
       fields.push_front(item);
       ref_pointer_array[el]= item;
       thd->change_item_tree(ref, new_item);

sql/item_func.cc

@@ -349,7 +349,7 @@ void Item_func::split_sum_func(THD *thd, Item **ref_pointer_array,
     else if (item->used_tables() || item->type() == SUM_FUNC_ITEM)
     {
       uint el= fields.elements;
-      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name, item);
       new_item->collation.set(item->collation);
       fields.push_front(item);
       ref_pointer_array[el]= item;

sql/item_row.cc

@@ -95,7 +95,8 @@ void Item_row::split_sum_func(THD *thd, Item **ref_pointer_array,
     else if ((*arg)->used_tables() || (*arg)->type() == SUM_FUNC_ITEM)
     {
       uint el= fields.elements;
-      Item *new_item= new Item_ref(ref_pointer_array + el, 0, (*arg)->name);
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, (*arg)->name,
+                                   *arg);
       fields.push_front(*arg);
       ref_pointer_array[el]= *arg;
       thd->change_item_tree(arg, new_item);

sql/item_strfunc.cc

@@ -1748,7 +1748,7 @@ void Item_func_make_set::split_sum_func(THD *thd, Item **ref_pointer_array,
   else if (item->used_tables() || item->type() == SUM_FUNC_ITEM)
   {
     uint el= fields.elements;
-    Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
+    Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name,item);
     fields.push_front(item);
     ref_pointer_array[el]= item;
     thd->change_item_tree(&item, new_item);