Memory leak with `NSLayoutConstraint`

I have a simple layout with a constraint connecting two views and I store that constraint in a strong property.

My expectation is that after removing one of those views, the constraint will still exist, but it will turn inactive. But it is not the case and my app crashes with

EXC_BAD_ACCESS

exception.

Luckily I am able to reproduce the issue. I attach the source code of the view controller. You can just paste it into a new project created with the "Single View App" template.

To reproduce the issue print the constraint description before and after removing one of the views. It happened to me once, that the constraint was not removed and the code worked (but it happened only once). With the debugger, I was able to check that the view I removed was not deallocated as well, while It already did not have a layer, so some parts of it were already deconstructed. After it happened I commented out the implementation of

printConstraintDescriptionButtonTouchedUpInside

and just set the breakpoint in the

removeViewButtonTouchedUpInside

. When the debugger paused the app after pressing the button for the second time the constraint did not exist anymore.

Is it disallowed to hold a strong reference to the

NSLayoutConstraint

instances? I have not found that information in the docs.

Compiled with Xcode Version 10.1 (10B61), running iOS Simulator iPhone SE 12.1. The same logic but not in the isolated snippet fails on the physical devices running iOS 12.1.3 (16D5032a) and 11.2.2 (15C202). Compiling with Xcode Version 9.4.1 does not change anything.

ViewController.m

#import "ViewController.h"

@interface ViewController ()

@property (strong, nonatomic) UIView* topView;
@property (strong, nonatomic) UIView* bottomView;

@property (strong, nonatomic) NSLayoutConstraint* constraint;

@property (strong, nonatomic) UIButton* removeViewButton;
@property (strong, nonatomic) UIButton* printConstraintDescriptionButton;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.topView = [UIView new];
    self.topView.backgroundColor = [UIColor grayColor];
    self.topView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:self.topView];

    self.bottomView = [UIView new];
    self.bottomView.backgroundColor = [UIColor grayColor];
    self.bottomView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:self.bottomView];

    self.removeViewButton = [UIButton buttonWithType:UIButtonTypeSystem];
    self.removeViewButton.translatesAutoresizingMaskIntoConstraints = NO;
    [self.removeViewButton setTitle:@"Remove View"
                           forState:UIControlStateNormal];
    [self.removeViewButton addTarget:self
                              action:@selector(removeViewButtonTouchedUpInside)
                    forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:self.removeViewButton];

    self.printConstraintDescriptionButton = [UIButton buttonWithType:UIButtonTypeSystem];
    self.printConstraintDescriptionButton.translatesAutoresizingMaskIntoConstraints = NO;
    [self.printConstraintDescriptionButton setTitle:@"Print Constraint Description"
                                           forState:UIControlStateNormal];
    [self.printConstraintDescriptionButton addTarget:self
                                              action:@selector(printConstraintDescriptionButtonTouchedUpInside)
                                    forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:self.printConstraintDescriptionButton];

    NSDictionary* views = @{
                            @"topView": self.topView,
                            @"bottomView": self.bottomView,
                            @"removeViewButton": self.removeViewButton,
                            @"printConstraintDescriptionButton": self.printConstraintDescriptionButton
    };

    NSArray<NSString*>* constraints = @[
                             @"H:|-[topView]-|",
                             @"H:|-[bottomView]-|",
                             @"H:|-[printConstraintDescriptionButton]-|",
                             @"H:|-[removeViewButton]-|",
                             @"V:|-[topView(==44)]",
                             @"V:[bottomView(==44)]",
                             @"V:[printConstraintDescriptionButton]-[removeViewButton]-|"
    ];

    [constraints enumerateObjectsUsingBlock:^(NSString * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:obj
                                                                          options:0
                                                                          metrics:nil
                                                                            views:views]];
    }];

    self.constraint = [NSLayoutConstraint constraintWithItem:self.topView
                                                   attribute:NSLayoutAttributeBottom
                                                   relatedBy:NSLayoutRelationEqual
                                                      toItem:self.bottomView
                                                   attribute:NSLayoutAttributeTop
                                                  multiplier:1.0
                                                    constant:-8.0];
    self.constraint.active = YES;
}

- (void)printConstraintDescriptionButtonTouchedUpInside {
    NSLog(@"%@", self.constraint);
}

- (void)removeViewButtonTouchedUpInside {
    [self.bottomView removeFromSuperview];
    self.bottomView = nil;
}

@end