Open Radar

 
Summary:
All UIButtons with no label nor images have an ambiguous internal layout, when displayed by a view controller embedded in a UINavigationController.

UINavigationTransitionView:0x7feb26c38560
|   UIViewControllerWrapperView:0x7feb26c1d6d0
|   |   •UIView:0x7feb26e26180
|   |   |   *<UILayoutGuide: 0x6000001b5540 - "UIViewSafeAreaLayoutGuide", layoutFrame = {{0, 64}, {1024, 704}}, owningView = <UIView: 0x7feb26e26180; frame = (0 0; 1024 768); autoresize = er = <CALayer: 0x6000006247e0>>>
|   |   |   *UIButton:0x7feb26e233e0
|   |   |   |   *UIButtonLabel:0x7feb26e1cc30- AMBIGUOUS LAYOUT for UIButtonLabel:0x7feb26e1cc30.minX{id: 240}, UIButtonLabel:0x7feb26e1cc30.minY{id: 241}
|   |   |   *UIButton:0x7feb26e26570
|   |   |   |   *UIButtonLabel:0x7feb26c0ef10- AMBIGUOUS LAYOUT for UIButtonLabel:0x7feb26c0ef10.minX{id: 238}, UIButtonLabel:0x7feb26c0ef10.minY{id: 239}

Steps to Reproduce:
1. Add a UINavigationController to your project
2. Add a UIViewController to your project, and set it to be the rootViewController of the UINavigationController
3. Add a UIButton to the view of your UIViewController
4. Set the UIButton `image` and `title` to nil
5. Layout the button in the view using autolayout
6. Run the app, break in the debugger, and use `po [[UIWindow keyWindow] _autolayoutTrace]` to show a description of the view hierarchy

Expected Results:
The _autolayoutTrace show no ambiguous layout

Actual Results:
_autolayoutTrace reveals that the layout for the private UIButtonLabel view (within the UIButton of step 3) is ambiguous

*UIButton:0x7feb26e233e0
|   *UIButtonLabel:0x7feb26e1cc30- AMBIGUOUS LAYOUT for UIButtonLabel:0x7feb26e1cc30.minX{id: 240}, UIButtonLabel:0x7feb26e1cc30.minY{id: 241}

Version:
11.0 (15A372)

Notes:
Attached is a minimal project reproducing the issue. Just run the project, pause and run `po [[UIWindow keyWindow] _autolayoutTrace]`.
The yellow button is set up using a storyboard, the red on using code. Both of them have an ambiguous internal layout.