This produces a circle, not a square

rect.swift

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let rectShape = CAShapeLayer()
        let bounds = CGRect(x: 400.0,
                            y: 400.0,
                            width: -200.0,
                            height: -200.0)

        let rectPath = UIBezierPath(roundedRect: bounds,
                                    byRoundingCorners: .allCorners,
                                    cornerRadii: .zero)

        rectShape.path = rectPath.cgPath
        rectShape.strokeColor = UIColor.black.cgColor
        rectShape.fillColor = UIColor.clear.cgColor
        rectShape.lineWidth = 3

        self.view.layer.addSublayer(rectShape)
    }

}

Produces: