&+(_:_:)

The overflow addition operator (&+) discards any bits that overflow the fixed width of the integer type. In the following example, the sum of 100 and 121 is greater than the maximum representable Int8 value, so the result is the partial value after discarding the overflowing bits.

let x: Int8 = 10 &+ 21
// x == 31
let y: Int8 = 100 &+ 121
// y == -35 (after overflow)

For more about arithmetic with overflow operators, see Overflow Operators in The Swift Programming Language.