title: Position and size are ambiguous for "Image view"
body: <p>I am trying to set a UIImageView has no fixed size and width in the storyboard. I set the leading space to container to greater than or equal to 20, top space to the container to  greater than or equal to 20 and trailing space to container to greater than or equal to 20 and vertical spacing to a UICollectionView, the UICollectionView satisfy the constraints. But the  UIImageView don't work, it's telling me it's Position and size are ambiguous. How can I make it work? My Idea is no matter how large the image view, it will always keep 20 space to leading, top, trailing and bottom.</p>
  <p>I have set the height and width to be greater than or equal to some value and their priority to 999, it's lower than the leading, top and trailing priority. and I have set the imageview to center horizontally. but i get height and vertical position are ambiguous for “Image view”.</p>
  <p><a href="https://i.stack.imgur.com/mOGt6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mOGt6.png" alt="" /></a></p>