title: Constraints on a UITableViewHeaderFooterView result in UIViewAlertForUnsatisfiableConstraints
body: <p>Been starring at this blankly for a bit now, probably too long, so I am trying a post here.</p>
  
  <p>Here's the error I am seeing:</p>
  
  <pre><code>2017-11-02 22:43:23.972361-0700 TableViewCellHeaderViewLayoutTest[88247:17250641] [LayoutConstraints] 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. 
  (
      "&lt;NSLayoutConstraint:0x60c000092930 V:|-(12)-[UILabel:0x7fd450c0eb80'Header 1']   (active, names: '|':_UITableViewHeaderFooterContentView:0x7fd450c0f290 )&gt;",
      "&lt;NSLayoutConstraint:0x60c0000927f0 UILabel:0x7fd450c0eb80'Header 1'.bottom == _UITableViewHeaderFooterContentView:0x7fd450c0f290.bottom - 6   (active)&gt;",
      "&lt;NSLayoutConstraint:0x60c000092ac0 'UIView-Encapsulated-Layout-Height' _UITableViewHeaderFooterContentView:0x7fd450c0f290.height == 17.5   (active)&gt;"
  )
  
  Will attempt to recover by breaking constraint 
  &lt;NSLayoutConstraint:0x60c0000927f0 UILabel:0x7fd450c0eb80'Header 1'.bottom == _UITableViewHeaderFooterContentView:0x7fd450c0f290.bottom - 6   (active)&gt;
  
  Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
  The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in &lt;UIKit/UIView.h&gt; may also be helpful.
  </code></pre>
  
  <p>It's a super simple implementation of my <code>UITableViewHeaderFooterView</code> subclass:</p>
  
  <pre><code>class TableViewSectionHeaderView: UITableViewHeaderFooterView {
  
      let titleLabel = UILabel()
  
      override init(reuseIdentifier: String?) {
          super.init(reuseIdentifier: reuseIdentifier)
  
          contentView.backgroundColor = .gray
  
          titleLabel.backgroundColor = contentView.backgroundColor
          titleLabel.font = UIFont.preferredFont(forTextStyle: .footnote)
  
          contentView.addSubview(titleLabel)
  
          titleLabel.translatesAutoresizingMaskIntoConstraints = false
          titleLabel.topAnchor.constraint(equalTo: contentView.topAnchor, constant: 12).isActive = true
          titleLabel.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: 6).isActive = true
          titleLabel.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -6).isActive = true
          titleLabel.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: -6).isActive = true
      }
  
  }
  </code></pre>
  
  <p>The thing that stands out to me is this constraint:</p>
  
  <pre><code>"&lt;NSLayoutConstraint:0x60c000092ac0 'UIView-Encapsulated-Layout-Height' _UITableViewHeaderFooterContentView:0x7fd450c0f290.height == 17.5   (active)&gt;"
  </code></pre>
  
  <p>If I inspect it it also has a <code>priority</code> of <code>1000</code>, which explains why my own constraints seems to fail.</p>
  
  <p>Everything lays out fine visually, but I am concerned about that warning. What am I doing wrong?</p>