Bug#11766424 59527: DECIMAL_BIN_SIZE: ASSERTION `SCALE >= 0 && PRECISION > 0 && SCALE <= PRE

Assertion happens due to missing initialization of unsigned_flag
for Item_func_set_user_var object. It leads to incorrect
calculation of decimal field size.
The fix is to add initialization of unsigned_flag.

mysql-test/r/variables.result

@@ -1547,6 +1547,22 @@ Warning	1292	Truncated incorrect key_cache_block_size value: '0'
 select @@max_long_data_size;
 @@max_long_data_size
 1048576
+#
+# Bug#11766424  59527: DECIMAL_BIN_SIZE: ASSERTION `SCALE >= 0 && PRECISION > 0 && SCALE <= PRE
+#
+CREATE TABLE t1(f1 DECIMAL(1,1) UNSIGNED);
+INSERT INTO t1 VALUES (0.2),(0.1);
+SELECT 1 FROM t1 GROUP BY @a:= (SELECT ROUND(f1) FROM t1 WHERE @a=f1);
+1
+1
+DROP TABLE t1;
+CREATE TABLE t1 AS SELECT @a:= CAST(1 AS UNSIGNED) AS a;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(1) unsigned NOT NULL DEFAULT '0'
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
 SET @@global.max_binlog_cache_size=DEFAULT;
 SET @@global.max_join_size=DEFAULT;
 SET @@global.key_buffer_size=@kbs;

mysql-test/t/variables.test

@@ -1298,6 +1298,19 @@ SET @@global.key_cache_block_size=0;
 #
 select @@max_long_data_size;
 
+--echo #
+--echo # Bug#11766424  59527: DECIMAL_BIN_SIZE: ASSERTION `SCALE >= 0 && PRECISION > 0 && SCALE <= PRE
+--echo #
+
+CREATE TABLE t1(f1 DECIMAL(1,1) UNSIGNED);
+INSERT INTO t1 VALUES (0.2),(0.1);
+SELECT 1 FROM t1 GROUP BY @a:= (SELECT ROUND(f1) FROM t1 WHERE @a=f1);
+DROP TABLE t1;
+
+CREATE TABLE t1 AS SELECT @a:= CAST(1 AS UNSIGNED) AS a;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
 # cleanup
 SET @@global.max_binlog_cache_size=DEFAULT;
 SET @@global.max_join_size=DEFAULT;

sql/item_func.cc

@@ -3840,6 +3840,7 @@ Item_func_set_user_var::fix_length_and_dec()
   maybe_null=args[0]->maybe_null;
   max_length=args[0]->max_length;
   decimals=args[0]->decimals;
+  unsigned_flag= args[0]->unsigned_flag;
   collation.set(args[0]->collation.collation, DERIVATION_IMPLICIT);
 }