Is it possible to replicate Swifts automatic numeric value bridging to Foundation (NSNumber) for (U)Int8/16/32/64 types?

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Yes (it’s possible): by conformance to protocol _ObjectiveCBridgeable

(The following answer is based on using Swift 2.2 and XCode 7.3.)

Just as I was pondering over whether to post or simply skip this question, I stumbled over swift/stdlib/public/core/BridgeObjectiveC.swift in the Swift source code, specifically the protocol _ObjectiveCBridgeable. I’ve briefly noticed the protocol previously at Swiftdoc.org, but in its current (empty) blueprint form in the latter, I’ve never given much thought to it. Using the blueprints for _ObjectiveCBridgeable from the Swift source we can, however, swiftly let some native of custom type conform to it.

Before proceeding, note that _ObjectiveCBridgeable is an internal/hidden protocol (_UnderScorePreFixedProtocol), so solutions based on it might break without warning in upcoming Swift versions.

Enabling Int64 bridging to Foundation class NSNumber

As an example, extend Int64 to conform to _ObjectiveCBridgeable, and subsequently test if this quite simple fix is sufficient for the implicit type conversion (bridging) from Int64 to Foundation class NSNumber holds. 

import Foundation

extension Int64: _ObjectiveCBridgeable {

    public typealias _ObjectiveCType = NSNumber

    public static func _isBridgedToObjectiveC() -> Bool {
        return true
    }

    public static func _getObjectiveCType() -> Any.Type {
        return _ObjectiveCType.self
    }

    public func _bridgeToObjectiveC() -> _ObjectiveCType {
        return NSNumber(longLong: self)
    }

    public static func _forceBridgeFromObjectiveC(source: _ObjectiveCType, inout result: Int64?) {
        result = source.longLongValue
    }

    public static func _conditionallyBridgeFromObjectiveC(source: _ObjectiveCType, inout result: Int64?) -> Bool {
        self._forceBridgeFromObjectiveC(source, result: &result)
        return true
    }
}

Test:

/* Test case: scalar */
let fooInt: Int = 42
let fooInt64: Int64 = 42
var fooAnyObj : AnyObject

fooAnyObj = fooInt    // OK, natively
fooAnyObj = fooInt64  // OK! _ObjectiveCBridgeable conformance successful

/* Test case: array */
let fooIntArr: [Int] = [42, 23]
let fooInt64Arr: [Int64] = [42, 23]
var fooAnyObjArr : [AnyObject]

fooAnyObjArr = fooIntArr    // OK, natively
fooAnyObjArr = fooInt64Arr  // OK! _ObjectiveCBridgeable conformance successful

Hence, conformance to _ObjectiveCBridgeable is indeed sufficient to enable automatic by-assignment bridging to the corresponding Foundation class; in this case, NSNumber (in Swift, __NSCFNumber).

Enabling Int8, UInt8, Int16, UInt16, Int32, UInt32, (Int64), and UInt64 bridging to NSNumber

The above conformance of Int64 to _ObjectiveCBridgeable can easily be modified to cover any of the Swift-native integer types, using the NSNumber conversion table below.

/* NSNumber initializer:               NSNumber native Swift type property
   --------------------------------    -----------------------------------
   init(char: <Int8>)                  .charValue
   init(unsignedChar: <UInt8>)         .unsignedCharValue
   init(short: <Int16>)                .shortValue
   init(unsignedShort: <UInt16>)       .unsignedShortValue
   init(int: <Int32>)                  .intValue
   init(unsignedInt: <UInt32>)         .unsignedIntValue
   init(longLong: <Int64>)             .longLongValue
   init(unsignedLongLong: <UInt64>)    .unsignedLongLongValue              */

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