+ is unavailable: Please use explicit type conversions or Strideable methods for mixed-type arithmetics

Rather than using the + operator, you can simply use Strideable.advanced(by:).

func increment<T: Strideable>(number: T) -> T {
    return number.advanced(by: 1)
}

increment(number: 2) //returns 3
increment(number: 2.5) //returns 3.5