value(forKey:) for NSObject not working in Swift 4

Question

Trying out Swift 4 in Xcode 9 Beta.

For some reason I'm getting a crash when using key value accessors on an NSObject.

Any ideas?

import Cocoa

class Person: NSObject {
    var name = ""
    var age = 0
}

let alpha = Person()
alpha.name = "Robert"
alpha.age = 53

alpha.value(forKey: "name")
// error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

Answer 1

Key-value coding requires Objective-C. In Swift 4 you have to mark members accessible from Obj-C explicitly:

@objcMembers
class Person: NSObject {

or

class Person: NSObject {
    @objc var name = ""
    var age = 0
}

See Swift Evolution 160

Answer 2

ObjC inference has been limited in Swift 4. You can mark your class with objcMembers for the compiler to generate ObjC-compliant accessors:

@objcMembers
class Person: NSObject {
    var name = ""
    var age = 0
}

Or get back the old behavior by setting Swift 3 Objc Inference to On in Build Settings: