How is this a reference to a var in concurrently executing code?

Using Swift
2

I can't explain why, but the error disappears if you capture the variable:

import os

@main
enum ScratchPad {
    static func main () {
        simple ()
    }
    
    static var lock = OSAllocatedUnfairLock ()

    static func simple() {
        var failures = [Int]()
        // lock.withLock {print (failures.count) }
        // Error: ^^^ Reference to captured var 'failures' in concurrently-executing code
        
        lock.withLock { [failures] in print (failures.count) }
        // Okay
    }
}

Or if you do this:

import os

@main
enum ScratchPad {

    static func main () {
        simple ()
    }
    
    static var lock = OSAllocatedUnfairLock ()

    class State {
        var failures = [Int]()
    }

    static func simple() {
        let state = State ()
        lock.withLock {print (state.failures.count) }
    }
}