date_formats.test:

  fix for bug #15828 after review
  doing val_str now before testing of null value secures the function for null values returned by dynamic functions - the fix before was incomplete andy covered constant null values

mysql-test/r/date_formats.result

@@ -470,3 +470,6 @@ NULL
 select str_to_date( NULL, 1 );
 str_to_date( NULL, 1 )
 NULL
+select str_to_date( 1, IF(1=1,NULL,NULL) );
+str_to_date( 1, IF(1=1,NULL,NULL) )
+NULL

mysql-test/t/date_formats.test

@@ -274,5 +274,5 @@ drop table t1;
 #
 select str_to_date( 1, NULL );
 select str_to_date( NULL, 1 );
-
+select str_to_date( 1, IF(1=1,NULL,NULL) );
 # End of 4.1 tests

sql/item_timefunc.cc

@@ -2750,9 +2750,9 @@ void Item_func_str_to_date::fix_length_and_dec()
   cached_field_type= MYSQL_TYPE_STRING;
   max_length= MAX_DATETIME_FULL_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
   cached_timestamp_type= MYSQL_TIMESTAMP_NONE;
+  format= args[1]->val_str(&format_str);
   if (!args[1]->null_value && (const_item= args[1]->const_item()))
   {
-    format= args[1]->val_str(&format_str);
     cached_format_type= get_date_time_result_type(format->ptr(),
                                                   format->length());
     switch (cached_format_type) {