布局约束冲突

Question

说，在xib文件中我添加了一个UIView，它的宽度和高度是固定的。在xib文件中没有添加LayoutConstraint。我将UIView与IBOutlet属性“contentView”连接，然后在viewDidLoad方法中执行：

- (void)viewDidLoad {

    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.

    contentView.translatesAutoresizingMaskIntoConstraints = NO;
    NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:contentView    attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0f constant:-20.0f];
    [self.view addConstraint:constraint];
}

当我运行程序时，似乎发生了约束冲突，但我不知道如何解决这个问题：

2014-10-14 09:37:57.436 TestAutoLayout[651:53850] 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. (Note: If you're seeing   NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the        UIView property translatesAutoresizingMaskIntoConstraints) 
(
"<NSIBPrototypingLayoutConstraint:0x7a028de0 'IB auto generated at build time for view with fixed frame' H:|-(0)-[UIView:0x7a029380](LTR)   (Names: '|':UIView:0x7a029470 )>",
"<NSIBPrototypingLayoutConstraint:0x7a028ed0 'IB auto generated at build time for view with fixed frame' H:[UIView:0x7a029380(320)]>",
"<NSLayoutConstraint:0x7a028620 UIView:0x7a029380.right == UIView:0x7a029470.right - 20>",
"<NSLayoutConstraint:0x7a02f600 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7a029470(320)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7a028620 UIView:0x7a029380.right == UIView:0x7a029470.right - 20> 

出于某种原因，我必须在程序中设置约束。谢谢。