Flattenable conforming entities define flattened, a function that flattens any arbitrary Optional, no matter how deeply nested, down to a single level of optionality.

Flattenable.swift

import XCTest

protocol Flattenable {
    func flattened() -> Any?
}

extension Swift.Optional : Flattenable {
    /// Flattens any arbitrary Optional, no matter how deeply nested, down to a single level of optionality.
    /// - SeeAlso: Really fun discussion going on here: https://forums.swift.org/t/challenge-flattening-nested-optionals/24083
    /// - Returns: A single-level Optional.
    func flattened() -> Any? {
        switch self {
        case .some(let x as Flattenable): return x.flattened()
        case .some(let x): return x
        case .none: return nil
        }
    }
}

final class FlattenableTests : XCTestCase {
    func testFlattened() {
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Int>.none.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Int>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Int>>.none?.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Int>>.some(Optional<Int>.none).flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Int>>.some(Optional<Int>.some(1)).flattened(), b: 1))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Int>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Int>>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Int>>>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Int>>>>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Optional<Int>>>>>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Optional<Optional<Int>>>>>>>.some(1).flattened(), b: Optional(1)))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Int>>>.none.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Int>>>>.none.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Int>>>>>.none.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Optional<Int>>>>>>.none.flattened(), b: nil))
        XCTAssert(isEqual(type: Optional<Int>.self, a: Optional<Optional<Optional<Optional<Optional<Optional<Optional<Int>>>>>>>.none.flattened(), b: nil))
        let _20levelsNested: Int???????????????????? = 20
        XCTAssert(isEqual(type: Optional<Int>.self, a: _20levelsNested.flattened(), b: Optional(20)))
    }

    // MARK: - Utilities
    /// Utility to compare `Any?` elements.
    private func isEqual<T : Equatable>(type: T.Type, a: Any?, b: Any?) -> Bool {
        guard let a = a as? T, let b = b as? T else { return false }
        return a == b
    }
}