Non-nil validation using KeyPath

Validate.swift

import Foundation

struct Test {

    var a: String?
    var b: Int?
    var c: NSObject?
    var x = 42

    enum Error: Swift.Error {
        case missingProperty(PartialKeyPath<Test>)
    }

    func validate(_ requiredProperties: Set<PartialKeyPath<Test>>) throws {
        for property in requiredProperties {
            let mirror = Mirror(reflecting: self[keyPath: property])
            guard mirror.displayStyle == .optional, mirror.children.isEmpty else { continue }
            throw Error.missingProperty(property)
        }
    }

}

var x = Test()
x.a = "foo"
//x.b = 42
x.c = NSObject()

do {
    try x.validate([ \Test.a, \Test.b, \Test.c, \Test.x ])
} catch Test.Error.missingProperty(let property) {
    print("\(property) not set") // Swift.WritableKeyPath<Test, Swift.Optional<Swift.Int>> not set
}