What happens when DB engine runs out of numbers to use for primary keys?

Question

Since DBs do not reuse numbers of deleted records it is possible to run out of numbers, especially if you pick not really a big integer type for this column.
What would

Answer 1

For MySQL, it is documented that:

The behavior of the auto-increment mechanism is not defined if a user assigns a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.