MDEV-22502 MDB crashes in CREATE TABLE AS SELECT when the precision of returning type = 0.

TRUNCATE(decimal_5_5) erroneously tried to create a DECIMAL(0,0) column.
Creating a DECIMAL(1,0) column instead.

mysql-test/r/func_math.result

@@ -994,6 +994,24 @@ SELECT -9223372036854775808 MOD -9223372036854775808;
 -9223372036854775808 MOD -9223372036854775808
 0
 #
+# MDEV-22502 MDB crashes in CREATE TABLE AS SELECT when the precision of returning type = 0
+#
+CREATE TABLE t1 (d decimal(5,5));
+INSERT INTO t1 VALUES (0.55555);
+SELECT TRUNCATE(d,0) FROM t1;
+TRUNCATE(d,0)
+0
+CREATE TABLE t2 AS SELECT TRUNCATE(d,0) FROM t1;
+SELECT * FROM t2;
+TRUNCATE(d,0)
+0
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `TRUNCATE(d,0)` decimal(1,0) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1, t2;
+#
 # MDEV-22503 MDB limits DECIMAL column precision to 16 doing CTAS with floor/ceil over DECIMAL(X,Y) where X > 16
 #
 CREATE TABLE t44 (d1 decimal(38,0) DEFAULT NULL);

mysql-test/t/func_math.test

@@ -719,6 +719,18 @@ SELECT  9223372036854775808 MOD -9223372036854775808;
 SELECT -9223372036854775808 MOD  9223372036854775808;
 SELECT -9223372036854775808 MOD -9223372036854775808;
 
+--echo #
+--echo # MDEV-22502 MDB crashes in CREATE TABLE AS SELECT when the precision of returning type = 0
+--echo #
+
+CREATE TABLE t1 (d decimal(5,5));
+INSERT INTO t1 VALUES (0.55555);
+SELECT TRUNCATE(d,0) FROM t1;
+CREATE TABLE t2 AS SELECT TRUNCATE(d,0) FROM t1;
+SELECT * FROM t2;
+SHOW CREATE TABLE t2;
+DROP TABLE t1, t2;
+
 
 --echo #
 --echo # MDEV-22503 MDB limits DECIMAL column precision to 16 doing CTAS with floor/ceil over DECIMAL(X,Y) where X > 16

sql/item_func.cc

@@ -2523,6 +2523,8 @@ void Item_func_round::fix_length_and_dec()
 
     precision-= decimals_delta - length_increase;
     decimals= MY_MIN(decimals_to_set, DECIMAL_MAX_SCALE);
+    if (!precision)
+      precision= 1; // DECIMAL(0,0) -> DECIMAL(1,0)
     max_length= my_decimal_precision_to_length_no_truncation(precision,
                                                              decimals,
                                                              unsigned_flag);