MDEV-22391 Assertion `0' failed in Item_type_holder::val_str on utf16 charset table query

Problem:
When handling a query like this:
  VALUES ('') UNION SELECT _utf16 0x0020 COLLATE utf16_bin;
Type_handler_string_result::Item_hybrid_func_fix_attributes()
tried to apply character set conversion Item_type_holder,
which causes a crash on DBUG_ASSERT(0) inside Item_type_holder::val_str().

Fix:
Overriding Item_type_holder's methods to avoid this, as follows:

  bool const_item() const { return false; }
  bool is_expensive() { return true; }

mysql-test/main/ctype_utf16.result

@@ -2814,3 +2814,28 @@ SET STORAGE_ENGINE=Default;
 #
 # End of 10.2 tests
 #
+#
+# Start of 10.3 tests
+#
+#
+# MDEV-22391 Assertion `0' failed in Item_type_holder::val_str on utf16 charset table query
+#
+SET NAMES utf8;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf16);
+SELECT * FROM (VALUES (1) UNION SELECT * FROM t1) AS t;
+1
+1
+DROP TABLE t1;
+VALUES (1) UNION SELECT _utf16 0x0020;
+1
+1
+ 
+VALUES ('') UNION SELECT _utf16 0x0020 COLLATE utf16_bin;
+
+
+VALUES ('') UNION VALUES ( _utf16 0x0020 COLLATE utf16_bin);
+
+
+#
+# End of 10.3 tests
+#

mysql-test/main/ctype_utf16.test

@@ -934,3 +934,24 @@ let $coll_pad='utf16_bin';
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-22391 Assertion `0' failed in Item_type_holder::val_str on utf16 charset table query
+--echo #
+
+SET NAMES utf8;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf16);
+SELECT * FROM (VALUES (1) UNION SELECT * FROM t1) AS t;
+DROP TABLE t1;
+VALUES (1) UNION SELECT _utf16 0x0020;
+VALUES ('') UNION SELECT _utf16 0x0020 COLLATE utf16_bin;
+VALUES ('') UNION VALUES ( _utf16 0x0020 COLLATE utf16_bin);
+
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

mysql-test/main/ctype_utf8.result

@@ -11321,5 +11321,17 @@ SELECT x AS 5天内最近一次登录时间 FROM t1;
 1
 DROP TABLE t1;
 #
+# MDEV-22391 Assertion `0' failed in Item_type_holder::val_str on utf16 charset table query
+#
+SET NAMES utf8;
+VALUES (_latin1 0xDF) UNION SELECT _utf8'a' COLLATE utf8_bin;
+_latin1 0xDF
+ß
+a
+VALUES (_latin1 0xDF) UNION VALUES(_utf8'a' COLLATE utf8_bin);
+_latin1 0xDF
+ß
+a
+#
 # End of 10.3 tests
 #

mysql-test/main/ctype_utf8.test

@@ -2256,6 +2256,13 @@ INSERT INTO t1 VALUES (1);
 SELECT x AS 5天内最近一次登录时间 FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-22391 Assertion `0' failed in Item_type_holder::val_str on utf16 charset table query
+--echo #
+
+SET NAMES utf8;
+VALUES (_latin1 0xDF) UNION SELECT _utf8'a' COLLATE utf8_bin;
+VALUES (_latin1 0xDF) UNION VALUES(_utf8'a' COLLATE utf8_bin);
 
 --echo #
 --echo # End of 10.3 tests

sql/item.h

@@ -6602,6 +6602,23 @@ class Item_type_holder: public Item,
 
   enum Type type() const { return TYPE_HOLDER; }
   TYPELIB *get_typelib() const { return enum_set_typelib; }
+  /*
+    When handling a query like this:
+      VALUES ('') UNION VALUES( _utf16 0x0020 COLLATE utf16_bin);
+    Item_type_holder can be passed to
+      Type_handler_xxx::Item_hybrid_func_fix_attributes()
+    We don't want the latter to perform character set conversion of a
+    Item_type_holder by calling its val_str(), which calls DBUG_ASSERT(0).
+    Let's override const_item() and is_expensive() to avoid this.
+    Note, Item_hybrid_func_fix_attributes() could probably
+    have a new argument to distinguish what we need:
+    - (a) aggregate data type attributes only
+    - (b) install converters after attribute aggregation
+    So st_select_lex_unit::join_union_type_attributes() could
+    ask it to do (a) only, without (b).
+  */
+  bool const_item() const { return false; }
+  bool is_expensive() { return true; }
   double val_real();
   longlong val_int();
   my_decimal *val_decimal(my_decimal *);