MDEV-12601 Hybrid functions create a column of an impossible type DOUBLE(256,4)

mysql-test/r/func_hybrid_type.result

@@ -3500,5 +3500,17 @@ t2	CREATE TABLE `t2` (
 DROP TABLE t2;
 DROP TABLE t1;
 #
+# MDEV-12601 Hybrid functions create a column of an impossible type DOUBLE(256,4)
+#
+CREATE TABLE t1 (a DOUBLE(255,4),b DOUBLE(255,3));
+CREATE TABLE t2 AS SELECT COALESCE(a,b) FROM t1;
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `COALESCE(a,b)` double(255,4) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t2;
+DROP TABLE t1;
+#
 # End of 10.3 tests
 #

mysql-test/t/func_hybrid_type.test

@@ -500,6 +500,16 @@ SHOW CREATE TABLE t2;
 DROP TABLE t2;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-12601 Hybrid functions create a column of an impossible type DOUBLE(256,4)
+--echo #
+
+CREATE TABLE t1 (a DOUBLE(255,4),b DOUBLE(255,3));
+CREATE TABLE t2 AS SELECT COALESCE(a,b) FROM t1;
+SHOW CREATE TABLE t2;
+DROP TABLE t2;
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 10.3 tests

sql/item_func.cc

@@ -662,6 +662,8 @@ void Item_func::count_real_length(Item **items, uint nitems)
     else
       max_length= length;
   }
+  // Corner case: COALESCE(DOUBLE(255,4), DOUBLE(255,3)) -> FLOAT(255, 4)
+  set_if_smaller(max_length, MAX_FIELD_CHARLENGTH);
 }