Size Classes and Stack Views Axis constraints conflicts - ios

Question

I'm trying something really simple and I do not understand the constraint conflict resulting. Let me explain:
I have a nice view controller with a full screen vertical UIStackView (contraints set to 0 to top, leading, trailing and bottom) with two views inside.
The top view as a constrained height of 200pt, the bottom view has no constraint. As the stack view has a Fill distribution mode, everything is fine.
Now, I want the stack view to switch to horizontal axis on landscape on iPhones. So, on my storyboard I select the wAny|hCompact size class, uninstall the height constraint, add a width constraint, and select horizontal axis...
The behavior works alright, unfortunately, it detects a constraint conflict I can't explain :
Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want.
Try this:
(1) look at each constraint and try to figure out which you don't expect;
(2) find the code that added the unwanted constraint or constraints and fix it.
(
"<NSLayoutConstraint:0x7fba62c1b7f0 H:[UIStackView:0x7fba62c0a3c0]-(0)-| (Names: '|':UIView:0x7fba62c0bd10 )>",
"<NSLayoutConstraint:0x7fba62c1b890 H:|-(0)-[UIStackView:0x7fba62c0a3c0] (Names: '|':UIView:0x7fba62c0bd10 )>",
"<NSLayoutConstraint:0x7fba62c1ba80 H:[UIView:0x7fba62c15420(200)]>",
"<NSLayoutConstraint:0x7fba62d1c230 'UISV-canvas-connection' UIStackView:0x7fba62c0a3c0.leading == UIView:0x7fba62c15420.leading>",
"<NSLayoutConstraint:0x7fba62dcad80 'UISV-canvas-connection' H:[UIView:0x7fba62c15420]-(0)-| (Names: '|':UIStackView:0x7fba62c0a3c0 )>",
"<NSLayoutConstraint:0x7fba62dc69d0 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7fba62c0bd10(736)]>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7fba62c1ba80 H:[UIView:0x7fba62c15420(200)]>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
So if someone can tell me what is wrong!

Answer 1

Ran into this problem and it appears to possibly be a bug in UIStackView where an internally generated constraint from the previous/default size class is lingering. This workaround (Swift) works for me:
override func traitCollectionDidChange(previousTraitCollection: UITraitCollection?) {
super.traitCollectionDidChange(previousTraitCollection)
self.setNeedsLayout()
self.layoutIfNeeded()
}