MDEV-23562 Assertion `time_type == MYSQL_TIMESTAMP_DATETIME' failed upon SELECT from versioned table

The code in vers_select_conds_t::init_from_sysvar() assumed that
the value of the system_versioning_asof is DATETIME.
But it also could be DATE after a query like this:
  SET system_versioning_asof = DATE(NOW());

Fixing Sys_var_vers_asof::update() to convert the value
of the assignment source to DATETIME if it returned DATE.

Now vers_select_conds_t::init_from_sysvar() always gets a DATETIME value.

Author: abarkov

mysql-test/suite/versioning/r/sysvars.result

@@ -186,4 +186,13 @@ SELECT @@global.system_versioning_asof;
 @@global.system_versioning_asof
 2002-01-01 00:00:00.000000
 SET @@global.system_versioning_asof= DEFAULT;
+#
+# MDEV-23562 Assertion `time_type == MYSQL_TIMESTAMP_DATETIME' failed upon SELECT from versioned table
+#
+CREATE TABLE t1 (a INT) WITH SYSTEM VERSIONING;
+SET system_versioning_asof= DATE(NOW());
+SELECT * FROM t1;
+a
+DROP TABLE t1;
+SET system_versioning_asof= DEFAULT;
 # End of 10.4 tests

mysql-test/suite/versioning/t/sysvars.test

@@ -138,4 +138,14 @@ SET @@global.system_versioning_asof= timestamp'2001-12-31 23:59:59.9999999';
 SELECT @@global.system_versioning_asof;
 SET @@global.system_versioning_asof= DEFAULT;
 
+--echo #
+--echo # MDEV-23562 Assertion `time_type == MYSQL_TIMESTAMP_DATETIME' failed upon SELECT from versioned table
+--echo #
+
+CREATE TABLE t1 (a INT) WITH SYSTEM VERSIONING;
+SET system_versioning_asof= DATE(NOW());
+SELECT * FROM t1;
+DROP TABLE t1;
+SET system_versioning_asof= DEFAULT;
+
 --echo # End of 10.4 tests

sql/sys_vars.ic

@@ -2669,7 +2669,11 @@ private:
         Datetime::Options opt(TIME_CONV_NONE |
                               TIME_NO_ZERO_IN_DATE |
                               TIME_NO_ZERO_DATE, thd);
-        res= var->value->get_date(thd, &out.ltime, opt);
+        /*
+          var->value is allowed to return DATETIME and DATE
+          Make sure to convert DATE to DATETIME.
+        */
+        res= Datetime(thd, var->value, opt).copy_to_mysql_time(&out.ltime);
       }
       else // set DEFAULT from global var
       {