clarify the order of evaluation for INSERT

mysql-test/r/default.result

@@ -3068,3 +3068,11 @@ t1	CREATE TABLE `t1` (
   `a` char(2) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci DEFAULT concat('A')
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
+create table t1 (a int default 1, b int default (1+1), c int);
+insert t1 (c) values (a);
+insert t1 (c) values (b);
+select * from t1;
+a	b	c
+1	2	1
+1	2	NULL
+drop table t1;

mysql-test/t/default.test

@@ -1840,3 +1840,12 @@ CREATE TABLE t1 (a VARCHAR(20) CHARACTER SET latin1 DEFAULT CONCAT('
 CREATE OR REPLACE TABLE t1 (a char(2) default concat('A') COLLATE utf8mb4_unicode_ci);
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
+
+#
+# Order of evaluation:
+#
+create table t1 (a int default 1, b int default (1+1), c int);
+insert t1 (c) values (a);
+insert t1 (c) values (b);
+select * from t1;
+drop table t1;

sql/item.cc

@@ -942,6 +942,7 @@ bool Item_field::register_field_in_write_map(void *arg)
 
   Fields are initialized in this order:
   - All fields that have default value as a constant are initialized first.
+  - Then user-specified values from the INSERT list
   - Then all fields that has a default expression, in field_index order.
   - Last all virtual fields, in field_index order.
 
@@ -949,6 +950,8 @@ bool Item_field::register_field_in_write_map(void *arg)
   - For default fields we can't access the same field or a field after
     itself that doesn't have a non-constant default value.
   - A virtual fields can't access itself or a virtual field after itself.
+  - user-specified values will not see virtual fields or default expressions,
+    as in INSERT t1 (a) VALUES (b);
 
   This is used by fix_vcol_expr() when a table is opened