Fixed bug #31471: decimal_bin_size: Assertion `scale >= 0 &&

                  precision > 0 && scale <= precision'.

A sign of a resulting item of the IFNULL function was not
updated and the maximal length of this result was calculated
improperly. Correct algorithm was copy&pasted from the IF
function implementation.

mysql-test/r/create.result

@@ -425,7 +425,7 @@ explain t2;
 Field	Type	Null	Key	Default	Extra
 a	int(11)	YES		NULL	
 b	bigint(11)	NO		0	
-c	bigint(11)	NO		0	
+c	bigint(11) unsigned	NO		0	
 d	date	YES		NULL	
 e	varchar(1)	NO			
 f	datetime	YES		NULL	

mysql-test/r/null.result

-drop table if exists t1;
+drop table if exists t1, t2;
 select null,\N,isnull(null),isnull(1/0),isnull(1/0 = null),ifnull(null,1),ifnull(null,"TRUE"),ifnull("TRUE","ERROR"),1/0 is null,1 is not null;
 NULL	NULL	isnull(null)	isnull(1/0)	isnull(1/0 = null)	ifnull(null,1)	ifnull(null,"TRUE")	ifnull("TRUE","ERROR")	1/0 is null	1 is not null
 NULL	NULL	1	1	1	1	TRUE	TRUE	1	1
@@ -320,3 +320,26 @@ bug19145c	CREATE TABLE `bug19145c` (
 drop table bug19145a;
 drop table bug19145b;
 drop table bug19145c;
+# End of 4.1 tests
+#
+# Bug #31471: decimal_bin_size: Assertion `scale >= 0 &&
+#             precision > 0 && scale <= precision'
+#
+CREATE TABLE t1 (a DECIMAL (1, 0) ZEROFILL, b DECIMAL (1, 0) ZEROFILL);
+INSERT INTO t1 (a, b) VALUES (0, 0);
+CREATE TABLE t2 SELECT IFNULL(a, b) FROM t1;
+DESCRIBE t2;
+Field	Type	Null	Key	Default	Extra
+IFNULL(a, b)	decimal(1,0) unsigned	YES		NULL	
+DROP TABLE t2;
+CREATE TABLE t2 SELECT IFNULL(a, NULL) FROM t1;
+DESCRIBE t2;
+Field	Type	Null	Key	Default	Extra
+IFNULL(a, NULL)	decimal(1,0)	YES		NULL	
+DROP TABLE t2;
+CREATE TABLE t2 SELECT IFNULL(NULL, b) FROM t1;
+DESCRIBE t2;
+Field	Type	Null	Key	Default	Extra
+IFNULL(NULL, b)	decimal(1,0)	YES		NULL	
+DROP TABLE t1, t2;
+# End of 5.0 tests

mysql-test/t/null.test

 # Initialise
 --disable_warnings
-drop table if exists t1;
+drop table if exists t1, t2;
 --enable_warnings
 
 #
@@ -231,4 +231,27 @@ drop table bug19145a;
 drop table bug19145b;
 drop table bug19145c;
 
-# End of 4.1 tests
+--echo # End of 4.1 tests
+
+--echo #
+--echo # Bug #31471: decimal_bin_size: Assertion `scale >= 0 &&
+--echo #             precision > 0 && scale <= precision'
+--echo #
+
+CREATE TABLE t1 (a DECIMAL (1, 0) ZEROFILL, b DECIMAL (1, 0) ZEROFILL);
+INSERT INTO t1 (a, b) VALUES (0, 0);
+
+CREATE TABLE t2 SELECT IFNULL(a, b) FROM t1;
+DESCRIBE t2;
+DROP TABLE t2;
+
+CREATE TABLE t2 SELECT IFNULL(a, NULL) FROM t1;
+DESCRIBE t2;
+DROP TABLE t2;
+
+CREATE TABLE t2 SELECT IFNULL(NULL, b) FROM t1;
+DESCRIBE t2;
+
+DROP TABLE t1, t2;
+
+--echo # End of 5.0 tests

sql/item_cmpfunc.cc

@@ -2020,10 +2020,20 @@ Item_func_ifnull::fix_length_and_dec()
   agg_result_type(&hybrid_type, args, 2);
   maybe_null=args[1]->maybe_null;
   decimals= max(args[0]->decimals, args[1]->decimals);
-  max_length= (hybrid_type == DECIMAL_RESULT || hybrid_type == INT_RESULT) ?
-    (max(args[0]->max_length - args[0]->decimals,
-         args[1]->max_length - args[1]->decimals) + decimals) :
-    max(args[0]->max_length, args[1]->max_length);
+  unsigned_flag= args[0]->unsigned_flag && args[1]->unsigned_flag;
+
+  if (hybrid_type == DECIMAL_RESULT || hybrid_type == INT_RESULT) 
+  {
+    int len0= args[0]->max_length - args[0]->decimals
+      - (args[0]->unsigned_flag ? 0 : 1);
+
+    int len1= args[1]->max_length - args[1]->decimals
+      - (args[1]->unsigned_flag ? 0 : 1);
+
+    max_length= max(len0, len1) + decimals + (unsigned_flag ? 0 : 1);
+  }
+  else
+    max_length= max(args[0]->max_length, args[1]->max_length);
 
   switch (hybrid_type) {
   case STRING_RESULT: