Fix for BUG#6825: When calculating Item_func_neg::max_length, add 1 for '-'.

For numeric constants we only need to add, since the parser doesn't produce 
negative numbers. 
For strings we only add (we actually could substract 1 if given string is a constant 
and it has '-number' form but we're not doing that because 
 * we set max_length bigger then necessary in other cases as well.  
 * the current solution is simpler and safer (bigger max_length is better then cutting out)

mysql-test/r/func_concat.result

@@ -32,3 +32,39 @@ select * from t1 where concat(A,C,B,D) = 'AAAA2003-03-011051';
 a	b	c	d
 AAAA	105	2003-03-01	1
 drop table t1;
+select 'a' union select concat('a', -4);
+a
+a
+a-4
+select 'a' union select concat('a', -4.5);
+a
+a
+a-4.5
+select 'a' union select concat('a', -(4 + 1));
+a
+a
+a-5
+select 'a' union select concat('a', 4 - 5);
+a
+a
+a-1
+select 'a' union select concat('a', -'3');
+a
+a
+a-3
+select 'a' union select concat('a', -concat('3',4));
+a
+a
+a-34
+select 'a' union select concat('a', -0);
+a
+a
+a0
+select 'a' union select concat('a', -0.0);
+a
+a
+a-0.0
+select 'a' union select concat('a', -0.0000);
+a
+a
+a-0.0000

mysql-test/r/metadata.result

@@ -3,7 +3,7 @@ select 1, 1.0, -1, "hello", NULL;
 Catalog	Database	Table	Table_alias	Column	Column_alias	Name	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
 def					1	8	1	1	N	32769	0	8
 def					1.0	5	3	3	N	32769	1	8
-def					-1	8	1	2	N	32769	0	8
+def					-1	8	2	2	N	32769	0	8
 def					hello	254	5	5	N	1	31	8
 def					NULL	6	0	0	Y	32896	0	63
 1	1.0	-1	hello	NULL

mysql-test/t/func_concat.test

@@ -34,3 +34,19 @@ create table t1 (a char(4), b double, c date, d tinyint(4));
 insert into t1 values ('AAAA', 105, '2003-03-01', 1);
 select * from t1 where concat(A,C,B,D) = 'AAAA2003-03-011051';
 drop table t1;
+
+# BUG#6825 
+select 'a' union select concat('a', -4);
+select 'a' union select concat('a', -4.5);
+
+select 'a' union select concat('a', -(4 + 1));
+select 'a' union select concat('a', 4 - 5);
+
+select 'a' union select concat('a', -'3');
+select 'a' union select concat('a', -concat('3',4));
+
+select 'a' union select concat('a', -0);
+select 'a' union select concat('a', -0.0);
+
+select 'a' union select concat('a', -0.0000);
+

sql/item_func.cc

@@ -775,9 +775,25 @@ longlong Item_func_neg::val_int()
 
 void Item_func_neg::fix_length_and_dec()
 {
+  enum Item_result arg_result= args[0]->result_type();
+  enum Item::Type  arg_type= args[0]->type();
   decimals=args[0]->decimals;
   max_length=args[0]->max_length;
   hybrid_type= REAL_RESULT;
+  
+  /*
+    We need to account for added '-' in the following cases:
+    A) argument is a real or integer positive constant - in this case 
+    argument's max_length is set to actual number of bytes occupied, and not 
+    maximum number of bytes real or integer may require. Note that all 
+    constants are non negative so we don't need to account for removed '-'.
+    B) argument returns a string.
+  */
+  if (arg_result == STRING_RESULT || 
+      (arg_type == REAL_ITEM && ((Item_real*)args[0])->value >= 0) ||
+      (arg_type == INT_ITEM && ((Item_int*)args[0])->value > 0))
+    max_length++;
+
   if (args[0]->result_type() == INT_RESULT)
   {
     /*