MDEV-37275 Cannot remove default value of NOT NULL column

Run-time has semantics duplication in unireg_check, default_value and
flags, so all three must be in sync before FRM creation. Special
unireg_check values for temporal field types was introduced by
32b28f9 WL#1266 "Separate auto-set logic from TIMESTAMP type."

Author: midenok

mysql-test/main/alter_table.result

@@ -3184,3 +3184,31 @@ DROP TABLE t;
 #
 # End of 10.7 tests
 #
+#
+# MDEV-37275 Cannot remove default value of NOT NULL column
+#
+create table t1 (d time default '00:00:00');
+alter table t1 alter column d drop default;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `d` time DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+create table t2 (d datetime not null default current_timestamp());
+alter table t2 alter column d drop default;
+show create table t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `d` datetime NOT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+create table t3 (d datetime not null);
+alter table t3 alter column d set default current_timestamp();
+show create table t3;
+Table	Create Table
+t3	CREATE TABLE `t3` (
+  `d` datetime NOT NULL DEFAULT current_timestamp()
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+drop tables t1, t2, t3;
+#
+# End of 10.11 tests
+#

mysql-test/main/alter_table.test

@@ -2462,3 +2462,25 @@ DROP TABLE t;
 --echo #
 --echo # End of 10.7 tests
 --echo #
+
+--echo #
+--echo # MDEV-37275 Cannot remove default value of NOT NULL column
+--echo #
+create table t1 (d time default '00:00:00');
+alter table t1 alter column d drop default;
+show create table t1;
+
+create table t2 (d datetime not null default current_timestamp());
+alter table t2 alter column d drop default;
+show create table t2;
+
+create table t3 (d datetime not null);
+alter table t3 alter column d set default current_timestamp();
+show create table t3;
+
+drop tables t1, t2, t3;
+
+--echo #
+--echo # End of 10.11 tests
+--echo #
+

sql/sql_table.cc

@@ -8711,6 +8711,9 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
         }
         else
         {
+          if (def->default_value != alter->default_value &&
+              def->has_default_now_unireg_check())
+            def->unireg_check= Field::NONE;
           if ((def->default_value= alter->default_value) ||
               !(def->flags & NOT_NULL_FLAG))
             def->flags&= ~NO_DEFAULT_VALUE_FLAG;