What is the ruling on wishing for non-existence, such as saying: "Oh, would that I had not been created"?

There is no harm in saying: "Would that I were nothing," or "Would that my mother had not given birth to me," if said out of fear of punishment. Such statements have been reported from some of the Salaf, among them Umar ibn al-Khattab and Abu Maysarah Amr ibn Shurahbil, who said it out of fear of Hellfire.