base

To find the position that corresponds with this index in the original, underlying collection, use that collection’s index(before:) method with the base property.

The following example declares a function that returns the index of the last even number in the passed array, if one is found. First, the function finds the position of the last even number as a ReversedIndex in a reversed view of the array of numbers. Next, the function calls the array’s index(before:) method to return the correct position in the passed array.

func indexOfLastEven(_ numbers: [Int]) -> Int? {
    let reversedNumbers = numbers.reversed()
    guard let i = reversedNumbers.firstIndex(where: { $0 % 2 == 0 })
        else { return nil }

    return numbers.index(before: i.base)
}

let numbers = [10, 20, 13, 19, 30, 52, 17, 40, 51]
if let lastEven = indexOfLastEven(numbers) {
    print("Last even number: \(numbers[lastEven])")
}
// Prints "Last even number: 40"