Fix:

CREATE TABLE t1 (s1 CHAR(5) CHARACTER SET latin1 COLLATE latin1_german2_ci);
INSERT INTO t1 VALUES ('Ü');
INSERT INTO t1 VALUES ('ue');
SELECT DISTINCT s1 FROM t1;

The above returned two rows in error.
Now it returns one row, in latin1_german2_ci:  Ü == ue

heap/hp_hash.c

@@ -371,8 +371,9 @@ int hp_rec_key_cmp(HP_KEYDEF *keydef, const byte *rec1, const byte *rec2)
     }
     if (seg->type == HA_KEYTYPE_TEXT)
     {
-      if (my_strnncoll(seg->charset,(uchar*) rec1+seg->start,seg->length,
-		       (uchar*) rec2+seg->start,seg->length))
+      if (seg->charset->coll->strnncollsp(seg->charset,
+      					  (uchar*) rec1+seg->start,seg->length,
+					  (uchar*) rec2+seg->start,seg->length))
 	return 1;
     }
     else
@@ -404,8 +405,9 @@ int hp_key_cmp(HP_KEYDEF *keydef, const byte *rec, const byte *key)
     }
     if (seg->type == HA_KEYTYPE_TEXT)
     {
-      if (my_strnncoll(seg->charset,(uchar*) rec+seg->start, seg->length,
-		       (uchar*) key, seg->length))
+      if (seg->charset->coll->strnncollsp(seg->charset,
+					  (uchar*) rec+seg->start, seg->length,
+					  (uchar*) key, seg->length))
 	return 1;
     }
     else

mysql-test/r/ctype_latin1_de.result

@@ -264,3 +264,18 @@ select * from t1 where word like CAST(0xDF as CHAR);
 word	word2
 	
 drop table t1;
+CREATE TABLE t1 (
+s1 CHAR(5) CHARACTER SET latin1 COLLATE latin1_german2_ci
+);
+INSERT INTO t1 VALUES ('');
+INSERT INTO t1 VALUES ('ue');
+SELECT DISTINCT s1 FROM t1;
+s1
+
+SELECT s1,COUNT(*) FROM t1 GROUP BY s1;
+s1	COUNT(*)
+	2
+SELECT COUNT(DISTINCT s1) FROM t1;
+COUNT(DISTINCT s1)
+1
+DROP TABLE t1;

mysql-test/t/ctype_latin1_de.test

@@ -73,3 +73,13 @@ select * from t1 where word like 'AE';
 select * from t1 where word like binary 0xDF;
 select * from t1 where word like CAST(0xDF as CHAR);
 drop table t1;
+
+CREATE TABLE t1 (
+s1 CHAR(5) CHARACTER SET latin1 COLLATE latin1_german2_ci
+);
+INSERT INTO t1 VALUES ('');
+INSERT INTO t1 VALUES ('ue');
+SELECT DISTINCT s1 FROM t1;
+SELECT s1,COUNT(*) FROM t1 GROUP BY s1;
+SELECT COUNT(DISTINCT s1) FROM t1;
+DROP TABLE t1;

sql/item_sum.cc

@@ -1023,7 +1023,9 @@ int simple_str_key_cmp(void* arg, byte* key1, byte* key2)
   Item_sum_count_distinct* item = (Item_sum_count_distinct*)arg;
   CHARSET_INFO *cs=item->key_charset;
   uint len=item->key_length;
-  return my_strnncoll(cs, (const uchar*) key1, len, (const uchar*) key2, len);
+  return cs->coll->strnncollsp(cs, 
+			       (const uchar*) key1, len, 
+			       (const uchar*) key2, len);
 }
 
 /*