MDEV-18447 Assertion `!is_zero_datetime()' failed in Timestamp_or_zero_datetime::tv

mysql-test/main/type_timestamp.result

@@ -1164,5 +1164,14 @@ NULL
 NULL
 DROP TABLE t1, t2, t3;
 #
+# MDEV-18447 Assertion `!is_zero_datetime()' failed in Timestamp_or_zero_datetime::tv
+#
+CREATE TABLE t1 (a TIMESTAMP DEFAULT 0, b TIMESTAMP DEFAULT 0, c TIME DEFAULT 0);
+INSERT INTO t1 VALUES (0,0,0);
+SELECT c IN (GREATEST(a,b)) FROM t1;
+c IN (GREATEST(a,b))
+0
+DROP TABLE t1;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_timestamp.test

@@ -760,6 +760,14 @@ CREATE TABLE t3 (pk INT PRIMARY KEY, b TIMESTAMP) ENGINE=MyISAM;
 SELECT ( SELECT b FROM t1 LIMIT 1 ) AS sq FROM t2 LEFT JOIN t3 USING (pk);
 DROP TABLE t1, t2, t3;
 
+--echo #
+--echo # MDEV-18447 Assertion `!is_zero_datetime()' failed in Timestamp_or_zero_datetime::tv
+--echo #
+
+CREATE TABLE t1 (a TIMESTAMP DEFAULT 0, b TIMESTAMP DEFAULT 0, c TIME DEFAULT 0);
+INSERT INTO t1 VALUES (0,0,0);
+SELECT c IN (GREATEST(a,b)) FROM t1;
+DROP TABLE t1;
 
 --echo #
 --echo # End of 10.4 tests

sql/sql_type.h

@@ -2435,6 +2435,12 @@ class Timestamp_or_zero_datetime_native:
       length(0); // safety
   }
   int save_in_field(Field *field, uint decimals) const;
+  Datetime to_datetime(THD *thd) const
+  {
+    return is_zero_datetime() ?
+           Datetime() :
+           Datetime(thd, Timestamp_or_zero_datetime(*this).tv());
+  }
   bool is_zero_datetime() const
   {
     return length() == 0;
@@ -2459,7 +2465,7 @@ class Timestamp_or_zero_datetime_native_null: public Timestamp_or_zero_datetime_
   Datetime to_datetime(THD *thd) const
   {
     return is_null() ? Datetime() :
-                       Datetime(thd, Timestamp_or_zero_datetime(*this).tv());
+                       Timestamp_or_zero_datetime_native::to_datetime(thd);
   }
   void to_TIME(THD *thd, MYSQL_TIME *to)
   {