MDEV-8256 A part of a ROW comparison is erroneously optimized away

Item_func_eq's created during conversion of a ROW equality to a conjunction
of scalar equalities did not set cmp_context for its arguments properly,
so some of these created Item_func_eq could be later erroneously eliminated.

mysql-test/r/ctype_utf8.result

@@ -9970,5 +9970,36 @@ SET NAMES utf8;
 SELECT * FROM `test😁😁test`;
 ERROR HY000: Invalid utf8 character string: 'test\xF0\x9F\x98\x81\xF0\x9F\x98\x81test'
 #
+#MDEV-8256 A part of a ROW comparison is erroneously optimized away
+#
+SET NAMES utf8;
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET utf8);
+INSERT INTO t1 VALUES ('1e1'),('1é1');
+SELECT * FROM t1 WHERE a=10;
+a
+1e1
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: '1é1'
+SELECT * FROM t1 WHERE a='1e1';
+a
+1e1
+1é1
+SELECT * FROM t1 WHERE a=10 AND a='1e1';
+a
+1e1
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: '1é1'
+SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+a
+1e1
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: '1é1'
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = 10) and (`test`.`t1`.`a` = '1e1'))
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/t/ctype_utf8.test

@@ -1789,6 +1789,20 @@ SET NAMES utf8;
 --error ER_INVALID_CHARACTER_STRING
 SELECT * FROM `test😁😁test`;
 
+--echo #
+--echo #MDEV-8256 A part of a ROW comparison is erroneously optimized away
+--echo #
+SET NAMES utf8;
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET utf8);
+INSERT INTO t1 VALUES ('1e1'),('1é1');
+SELECT * FROM t1 WHERE a=10;
+SELECT * FROM t1 WHERE a='1e1';
+SELECT * FROM t1 WHERE a=10 AND a='1e1';
+SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item_cmpfunc.h

@@ -325,6 +325,14 @@ class Item_bool_rowready_func2 :public Item_bool_func2
   {
     return cmp.set_cmp_func(this, tmp_arg, tmp_arg + 1, true);
   }
+  bool set_cmp_func_and_arg_cmp_context()
+  {
+    if (set_cmp_func())
+      return true;
+    tmp_arg[0]->cmp_context= tmp_arg[1]->cmp_context=
+      item_cmp_type(tmp_arg[0]->result_type(), tmp_arg[1]->result_type());
+    return false;
+  }
   CHARSET_INFO *compare_collation() const
   { return cmp.cmp_collation.collation; }
   Arg_comparator *get_comparator() { return &cmp; }

sql/sql_select.cc

@@ -12566,7 +12566,7 @@ static bool check_simple_equality(THD *thd, Item *left_item, Item *right_item,
           Item_func_eq *eq_item;
           if (!(eq_item= new (thd->mem_root) Item_func_eq(orig_left_item,
                                                           orig_right_item)) ||
-              eq_item->set_cmp_func())
+              eq_item->set_cmp_func_and_arg_cmp_context())
             return FALSE;
           eq_item->quick_fix_field();
           item= eq_item;
@@ -12660,7 +12660,7 @@ static bool check_row_equality(THD *thd, Item *left_row, Item_row *right_row,
     {
       Item_func_eq *eq_item;
       if (!(eq_item= new Item_func_eq(left_item, right_item)) ||
-          eq_item->set_cmp_func())
+          eq_item->set_cmp_func_and_arg_cmp_context())
         return FALSE;
       eq_item->quick_fix_field();
       eq_list->push_back(eq_item);