Fix for BUG#6976:

In Item_ref::Item_ref set maybe_null (and other fields fix_fields sets) to be the 
same as in (*ref), because Item_ref::fix_fields() will not be called.  Previously 
maybe_null was 0 always and this produced a bogus state where 
maybe_null==0 && is_null() == true 
which broke evaluation for some upper-level Items, like AND and OR.

mysql-test/r/group_by.result

@@ -626,3 +626,15 @@ explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ALL	NULL	NULL	NULL	NULL	6	Using filesort
 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

@@ -447,3 +447,12 @@ INSERT INTO t1 VALUES (1,2),(2,3),(4,5),(3,5),(1,5),(23,5);
 SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 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

@@ -687,6 +687,7 @@ bool Item_null::send(THD *thd, String *packet)
 /*
   This is used for HAVING clause
   Find field in select list having the same name
+  This is not always called, see also Item_ref::Item_ref
  */
 
 bool Item_ref::fix_fields(THD *thd,TABLE_LIST *tables)

sql/item.h

@@ -401,7 +401,19 @@ public:
   Item_ref(char *db_par,char *table_name_par,char *field_name_par)
     :Item_ident(db_par,table_name_par,field_name_par),ref(0) {}
   Item_ref(Item **item, char *table_name_par,char *field_name_par)
-    :Item_ident(NullS,table_name_par,field_name_par),ref(item) {}
+    :Item_ident(NullS,table_name_par,field_name_par),ref(item)
+  {
+    /*
+      This ctor is called from Item_XXX::split_sum_func, and fix_fields will
+      not be called for *this, so we must setup everything here. **ref is 
+      already fixed at this point.
+    */
+    max_length= (*ref)->max_length;
+    decimals=	(*ref)->decimals;
+    binary=	(*ref)->binary;
+    with_sum_func= (*ref)->with_sum_func;
+    maybe_null= (*ref)->maybe_null;
+  }
   enum Type type() const		{ return REF_ITEM; }
   bool eq(const Item *item, bool binary_cmp) const
   { return (*ref)->eq(item, binary_cmp); }