MDEV-13596 CHECK constraints disallow NULL to pass through, violating SQL

SQL Standard (4.23.3.4 Table check constraints, part 2, SQL:2016) says
that CHECK constraint rejects rows *only* if the condition is FALSE.
That is, both TRUE and NULL should be allowed.

mysql-test/r/check_constraint.result

@@ -142,3 +142,13 @@ create table t1 (a int check (@b in (select user from mysql.user)));
 ERROR HY000: Function or expression 'select ...' cannot be used in the CHECK clause of `a`
 create table t1 (a int check (a > @b));
 ERROR HY000: Function or expression '@b' cannot be used in the CHECK clause of `a`
+create table t1 (a int check (a = 1));
+insert t1 values (1);
+insert t1 values (2);
+ERROR 23000: CONSTRAINT `a` failed for `test`.`t1`
+insert t1 values (NULL);
+select * from t1;
+a
+1
+NULL
+drop table t1;

mysql-test/t/check_constraint.test

@@ -92,3 +92,14 @@ create or replace table t1( c1 int auto_increment primary key, check( c1 > 0 or
 create table t1 (a int check (@b in (select user from mysql.user)));
 --error ER_GENERATED_COLUMN_FUNCTION_IS_NOT_ALLOWED
 create table t1 (a int check (a > @b));
+
+#
+# MDEV-13596 CHECK constraints disallow NULL to pass through, violating SQL
+#
+create table t1 (a int check (a = 1));
+insert t1 values (1);
+--error ER_CONSTRAINT_FAILED
+insert t1 values (2);
+insert t1 values (NULL);
+select * from t1;
+drop table t1;

sql/table.cc

@@ -5090,7 +5090,8 @@ int TABLE::verify_constraints(bool ignore_failure)
   {
     for (Virtual_column_info **chk= check_constraints ; *chk ; chk++)
     {
-      if ((*chk)->expr->val_int() == 0)
+      /* yes! NULL is ok, see 4.23.3.4 Table check constraints, part 2, SQL:2016 */
+      if ((*chk)->expr->val_int() == 0 && !(*chk)->expr->null_value)
       {
         my_error(ER_CONSTRAINT_FAILED,
                  MYF(ignore_failure ? ME_JUST_WARNING : 0), (*chk)->name.str,