Bug#35710183: mysql server 8.1.0 crashed at Item_ident::walk

While trying to clone a Item_ref object during condition
pushdown, Item_ref::print does not print the underlying
expression since it is an alias. However, in this case
the underlying expression is a user variable, which
results in a mismatch for the number of fields present
in the original expression and the cloned expression
(parser creates a field object for the alias in the
cloned expression while the original expression has
none).
In case of condition pushdown, we always want the
original expression to be printed instead of the alias.
So the solution is to skip the code path which has
special handling for aliases. Also, removed a
(!const_item()) check which was added for a similar
case but for a constant expression.

Change-Id: I3b390915dbc7618175fd1b65e06af73eb0a9f6ef

Author: Chaithra Gopalareddy

mysql-test/r/derived.result

@@ -4051,7 +4051,7 @@ id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered
 1	SIMPLE	t1	NULL	system	NULL	NULL	NULL	NULL	1	100.00	NULL
 Warnings:
 Note	1249	Select 3 was reduced during optimization
-Note	1003	/* select#1 */ select 1 AS `alias1`,1 AS `alias2` from dual
+Note	1003	/* select#1 */ select 1 AS `alias1`,`alias1` AS `alias2` from dual
 SELECT *
 FROM (SELECT (1) alias1,
 (SELECT alias1) alias2
@@ -4079,7 +4079,7 @@ id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered
 1	SIMPLE	t1	NULL	system	NULL	NULL	NULL	NULL	1	100.00	NULL
 Warnings:
 Note	1249	Select 3 was reduced during optimization
-Note	1003	/* select#1 */ select 1 AS `alias2` from dual
+Note	1003	/* select#1 */ select `alias1` AS `alias2` from dual
 SELECT alias2
 FROM (SELECT (1) alias1,
 (SELECT alias1) alias2

sql/item.cc

@@ -8372,7 +8372,7 @@ void Item_ref::print(const THD *thd, String *str,
   if (m_ref_item == nullptr)  // Unresolved reference: print reference
     return Item_ident::print(thd, str, query_type);
 
-  if (!const_item() && m_alias_of_expr &&
+  if (!thd->lex->reparse_derived_table_condition && m_alias_of_expr &&
       ref_item()->type() != Item::CACHE_ITEM && ref_type() != VIEW_REF &&
       table_name == nullptr && item_name.ptr()) {
     Simple_cstring str1 = ref_item()->real_item()->item_name;