Open Radar

 
Summary:
With Auto Layout enabled topLayoutGuide and bottomLayoutGuide don't have correct horizontal constraints. I don't think it impacts the layout process, but it makes debugging ambiguous layouts harder.

Steps to Reproduce:
1. Create a new Single View Application project
2. Build and run it
3. Pause the program execution
4. In the lldb type:
po [[UIWindow keyWindow] _autolayoutTrace]
In my case it prints:
*<UIWindow:0x8e60e00> - AMBIGUOUS LAYOUT
|   *<UIView:0x8e62270>
|   |   *<_UILayoutGuide:0x8e62450> - AMBIGUOUS LAYOUT
|   |   *<_UILayoutGuide:0x8e627f0> - AMBIGUOUS LAYOUT

5. Now use memory addresses of the layout guides to check their constraints with:
po [0x8e62450 constraintsAffectingLayoutForAxis:0]
It returns an empty array of horizontal constraints. It's the same for the other layout guide.

6. It also happens when Storyboards aren't used and everything is laid out in code.

Expected Results:
Top and bottom layout guides shouldn't be ambiguous.

Actual Results:
Top and bottom layout guides are ambiguous horizontally. They don't have any horizontal constraints set up.

Notes:
One workaround that I've found is to add:

    NSDictionary *viewsDict = @{@"topLayoutGuide" : self.topLayoutGuide, @"bottomLayoutGuide" : self.bottomLayoutGuide};
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[topLayoutGuide]|" options:0 metrics:nil views:viewsDict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bottomLayoutGuide]|" options:0 metrics:nil views:viewsDict]];

in viewDidLoad/loadView.